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Chapter 5 Types of Compounds

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					       5
      CHAPTER
                    Types of
                    Compounds




       Chapter Preview
       Sections                                            Covalent Crops


                                               F
       5.1 Ionic Compounds
            MiniLab 5.1 A Chemical Weather          armers often spray chemical compounds on
                        Predictor                   their crops as fertilizers, providing added
       5.2 Molecular Substances                     nutrients, or as pesticides, to reduce crop
            MiniLab 5.2 Where’s the Calcium?
            ChemLab Ionic or Covalent?         destruction by insects. Most compounds, includ-
                                               ing those in living organisms, are covalently
                                               bonded. Fertilizers and pesticides used on crops
                                               are covalently bonded.



152
                                       Start-up Activities

                                                          What I Already Know
Elements, Compounds, and                                  Review the following concepts
Mixtures                                                  before studying this chapter.
What are some differences among elements,                 Chapter 2: arrangement of electrons
compounds, and mixtures?                                  in an atom
                                                          Chapter 3: periodicity of electron
Safety Precautions                                        arrangements in atoms; importance
                                                          of valence electrons
                                                          Chapter 4: formation of ionic com-
                                                          pounds, formation of covalent
Materials                                                 compounds
• plastic freezer bag containing the following
  labeled items:
copper wire                   chalk (calcium carbonate)   Reading Chemistry
small package of salt         piece of granite
                                                          Carefully read the steps about how to
sugar water in a vial         pencil
                                                          write a chemical formula on page 158.
Procedure                                                 Next, scan the different tables
                                                          throughout the chapter. Write down
1. Construct a data table and use it to record your       the formulas for substances that
   observations.                                          appear often. Try to determine the
2. Obtain a bag of objects. Identify each object and      elements in their makeup.
   classsify it as an element, compound, heterogeneous
   mixture, or homogeneous mixture. The elements
   appear in the periodic table.

Analysis
1. If you know the name of a substance, how can you       Preview this chapter’s content and
   find out whether or not it is an element?              activities at chemistryca.com
2. Examine the contents of your refrigerator at home.
   Classify what you find as elements, compounds, or
   mixtures.




                                                                                                  153
            SECTION

            5.1                     Ionic Compounds
                                    S
                                         eawater contains many dissolved substances, mostly dissolved sodium
 SECTION PREVIEW                         chloride. In Chapter 4, you learned that sodium chloride is an ionic com-
                                         pound. Another ionic compound found dissolved in seawater is magne-
 Objectives                         sium chloride. Some common
                                    ionic compounds used in every-
 ✓ Apply ionic charge
                                    day life are potassium chloride, a
 to writing formulas
 for ionic compounds.               salt substitute used by people
                                    avoiding sodium for health
 ✓ Apply formulas
                                    reasons; potassium iodide,
 to name ionic
 compounds.                         added to table salt to prevent
                                    iodine deficiency; and sodium
 ✓ Interpret the infor-
                                    fluoride, added to many
 mation in a chemical
 formula.                           toothpastes to strengthen
                                    tooth enamel. You will
 Review Vocabulary                  learn how to use the language
 Ion: an atom or group              of chemistry to name and
 of combined atoms                  write the formulas of ionic
 that has a charge                  compounds.
 because of the loss or
 gain of electrons.
                                    Formulas and Names of Ionic Compounds
 New Vocabulary
 binary compound                       Recall from Chapter 4 that the submicroscopic structure of ionic com-
 formula unit                       pounds helps explain why they share certain macroscopic properties such
 oxidation number                   as high melting points, brittleness, and the ability to conduct electricity
 polyatomic ion                     when molten or when dissolved in water. What is it about the structure of
 hydrate                            these compounds that gives them properties such as the one shown in
 hygroscopic                        Figure 5.1? The answer involves the ions of which they are made.
 deliquescent                          You have learned that ionic compounds are made up of oppositely
 anhydrous                                                               charged ions held together strongly in
                                                                         well-organized units. Because of their
                                                                         structure, they usually are hard solids
                                                                         at room temperature and are difficult

                                                                         Figure 5.1
                                                                         Humpty Dumpty’s Downfall
                                                                         Eggshells are made mostly of ionic com-
                                                                         pounds such as calcium phosphate,
                                                                         Ca3(PO4)2 , which makes them brittle.
                                                                         When broken, eggshells shatter into many
                                                                         pieces that can’t be put together again.

154           Chapter 5   Types of Compounds
                                                                                 Figure 5.2
                                                                                 Repeating Units
                                                                                 The structure of magnesium
                                                                                 oxide is a repeating pattern of
                                                                                 magnesium and oxide ions.
                                                                                 Each Mg2 ion is surrounded
                                                                                 by six O2 ions, which, in turn,
                                                                                 are each surrounded by six
                                                                                 Mg2 ions. The structure as a
                                                                                 whole is neutral. In the dia-
                                                                                 gram on the left, the structure
                                                                                 is expanded so you can better
                                                                                 see the geometric arrange-
                                O2           Mg2                                 ment.


to melt. Look at the structure of magnesium oxide in Figure 5.2. When
ionic compounds melt or dissolve in water, their three-dimensional struc-
ture breaks apart, and the ions are released from the structure. These
charged ions are now free to move and can conduct an electrical current.

Binary Ionic Compounds
   Formulas are part of the language that is used to communicate infor-
mation about substances. As a first step in studying this new language,
you will learn how to name and write formulas for ionic compounds.
   Sodium chloride (NaCl) contains only sodium and chlorine, and potas-
sium iodide (KI) contains only potassium and iodine. Each is an example                    binary:
of a binary compound, which is a compound that contains only two ele-                       bini (L) two by
                                                                                            two
ments. Binary ionic compounds can contain more than one ion of each
                                                                                           Binary compounds
element, as in CaF2, but they are not composed of three or more different
                                                                                           contain two, and
elements, as are more complex compounds.                                                   only two, elements.
   To name a binary ionic compound, first write the name of the positive-
ly charged ion, usually a metal, and then add the name of the nonmetal or
negatively charged ion, whose name has been modified to end in -ide. The
compound formed from potassium and chlorine is called potassium chlo-
ride. Magnesium combines with oxygen to form a compound called mag-
nesium oxide.
   You are already familiar with one formula for an ionic compound—
NaCl. Sodium chloride contains sodium ions that have a 1 charge and
chloride ions that have a 1 charge. You have learned that compounds are
electrically neutral. This means that the sum of the charges in an ionic
compound must always equal zero. Thus, one Na balances one Cl in
sodium chloride. When you write a formula, you add subscripts to the
symbols for the ions until the algebraic sum of the ions’ charges is zero.
The smallest subscript to both ions that results in a total charge of zero
is 1. However, no subscript needs to be written because it is understood
that only one ion or atom of an element is present if there is no subscript.
The formula NaCl indicates that sodium chloride contains sodium and
chloride ions, that there is one sodium ion present for every chloride ion
in the compound, and that the compound has no overall charge.

                                                                               5.1   Ionic Compounds             155
                                        If more than one ion of a given element is present in a compound, the
                                     subscript indicates how many ions are present. The mineral known as fluo-
                                     rite is calcium fluoride, which has the formula CaF2. This formula indicates
                                     that there is one calcium ion for every two fluoride ions in the compound.
  Rain falling over or near          In an ionic compound, a formula represents the smallest ratio of atoms or
  an ocean is often salty
  because bits of salt are
                                     ions in the compound. In a covalent compound, the smallest unit of the
  picked up by the wind              compound is a molecule, so a formula represents a single molecule of a
  after bubbles of sea               compound. However, ionic compounds do not form molecules. Their
  foam burst apart. This             structures are repeating patterns of ions, as Figure 5.3 shows. Should the
  salt can remain sus-               formula of calcium fluoride be written as CaF2, Ca2F4, or even Ca3F6? A
  pended in the air for              properly written formula has the simplest possible ratio of the ions pres-
  some time before set-
  tling. Crops raised in
                                     ent. This simplest ratio of ions in a compound is called a formula unit.
  some coastal areas—                Each formula unit of calcium fluoride consists of one calcium ion and two
  such as the famous arti-           fluoride ions. Each of the three ions has a stable octet configuration of
  choke fields south of              electrons, and the formula unit has no overall charge. Although the sum of
  San Francisco—often                the ionic charges in both CaF2 and Ca2F4 is zero, only CaF2 is a correct for-
  have a distinctive and             mula. One formula unit of calcium fluoride has the formula CaF2.
  highly sought-after fla-
  vor because of the salty
  rainfall. Salt spread in           Predicting Charge on Ions
  this way also contrib-
  utes to corrosion of                  You have studied ionic compounds in which sodium becomes a posi-
  metal objects, especially          tive ion with a single positive charge and calcium becomes a positive ion
  automobiles.                       with two positive charges. Examine the periodic table to see if there is
                                     a way to predict the charge that different elements will have when they
                                     become ions. Which elements will lose electrons and which will gain
                                     electrons?
                                        The noble gases each have eight electrons in their outer-energy levels.
Figure 5.3
                                     Metals have few outer-level electrons so they tend to lose them and become
The Formula for Calcium
                                     positive ions. Sodium must lose just one electron, becoming an Na ion.
Fluoride
When calcium fluoride forms
                                     Calcium must lose two electrons, becoming a Ca2 ion. Most nonmetals,
from Ca and F, the two               on the other hand, have outer-energy levels that contain four to seven elec-
valence electrons from calci-        trons, so they tend to gain electrons and become negative ions. Trace the
um are transferred to two flu-       gain and loss of electrons in the example shown in Figure 5.4.
orine atoms, leaving the Ca
with a 2 charge and each F
                                                            2+           –       –
with a 1 charge.                     Ca         F   F ˇ [Ca]         F       F


                                                                                     An ionic bond forms between the
                                                                                     positive Ca2 ion and each negative
                                                                                     F ion. Although there are many
                                                                                     Ca2 and F ions in a crystal of CaF2 ,
                                                                                     one formula unit of CaF2 contains
                                                                                     one Ca2 ion and two F ions.




                                                    Ca2          F


156            Chapter 5   Types of Compounds
                        Figure 5.4
                        Lime Is Calcium Oxide
                        The compound commonly called lime is calcium oxide. It
                        is used to make steel and cement and is added to acidic
                        lakes and soil to neutralize the effects of acidity.

   Do you recognize a periodic trend in ionic charges? For the elements in
the main groups of the periodic table—Groups 1, 2, and 13 through 18—
group numbers can be used to predict these charges. Because all elements
in a given group have the same number of electrons in their outer-energy
level, they must lose or gain the same number of electrons to achieve a
noble-gas electron configuration. Metals always lose electrons and non-
metals always gain electrons when they form ions. The charge on the ion
is known as the oxidation number of the atom. The oxidation numbers
for many elements in the main groups are arranged by group number in                   Ca ˇ Ca2+ O ˇ O 2–
Table 5.1. Oxidation numbers for elements in Groups 3 through 12, the
transition elements, cannot be predicted by group number.
                                                                                        Ca 2+ O 2– ˇ CaO
   Alumina is the common name for aluminum oxide. It is used to pro-                       Calcium is a metal that
duce aluminum metal, to make sandpaper and other abrasives, and to                         loses two electrons to
separate mixtures of chemicals by a technique called chromatography.                       become a Ca2 ion; oxygen
Aluminum is in Group 13, so it loses its three outer electrons to become                   is a nonmetal that must
an Al3 ion; oxygen is in Group 16 and has six valence electrons, so it                     gain two electrons to
gains two electrons to become an O2 ion.                                                   achieve the stable octet of
                                                                                           the noble gas neon, so it
                        Al     O ˇ Al3+     O   2–
                                                                                           becomes an O2 ion.
Notice that one of aluminum’s three electrons has not been taken up by                     Because a formula unit
the oxygen atom. Because all the electrons must be accounted for, more                     must be neutral, one Ca2
than one oxygen atom must be involved in the reaction. But, oxygen can-                    ion can combine with
not gain only one electron, so a second aluminum atom must be present to                   only one O2 ion. The for-
                                                                                           mula for calcium oxide is
contribute a second electron to oxygen. In all, two Al3 ions must combine
                                                                                           CaO.
with three O2 ions to form Al2O3. Remember that the charges in the for-
mula for aluminum oxide must add up to zero.
                 2Al3        3O2 ˇ Al2O3
       Al   Al    O      O     O ˇ Al3+ Al3+         O   2–
                                                                 O   2–
                                                                            O   2–




  Table 5.1 Ionic Charges of Representative Elements

  Group Number            Oxidation Number                    Examples
  Metals
  11                                 1                        Li , Na , K
  12                                 2                        Mg2 , Ca2
  13                                 3                          B3 , Al3
  Nonmetals
  15                                 3                      N3 , P3
  16                                 2                       O2 , S2
  17                                 1                   F , Cl , Br , I


                                                                                     5.1   Ionic Compounds        157
  SAMPLE PROBLEM                      1        Writing a Simple Formula

                                                Write the formula for an ionic compound containing sodium and
                                              sulfur.
                            Analyze       • in Groupis16 and has1, sooxidation number ofnumber of 1 . Sulfur is
                                            Sodium in Group
                                                                    an
                                                                       it has an oxidation
                                                                                           2 .
                             Set Up       • Write the symbols for sodium and sulfur ions in formula form, placing
                                            the positive ion first.
                                                                          Na S2
                              Solve       •   The formula as written has one positive charge and two negative
                                              charges. To maintain neutrality, one more positive charge is needed to
                                              balance the 2 charge. This is accomplished by adding a second sodi-
                                              um ion and is indicated by placing the subscript 2 after the symbol for
                                              sodium in the formula. The correct formula is then written as Na2S.
                             Check        •   Check to be sure that you have not changed the charges of the ions
                                              and that the overall charge of the formula is zero.
                                                  2(1 ) (2 ) 0 The formula as written is correct.

  PRACTICE PROBLEMS

                                              1. Write the formula for each of the following compounds.
                                                 a) lithium oxide
                                                 b) calcium bromide
For more practice with solving                   c) sodium oxide
problems, see Supplemental
Practice Problems,
                                                 d) aluminum sulfide
Appendix B.                                   2. Write the formula for the compound formed from each of the
                                                 following pairs of elements.
                                                 a) barium and oxygen
                                                 b) strontium and iodine
                                                 c) lithium and chlorine
                                                 d) radium and chlorine




                                        Compounds Containing Polyatomic Ions
                                           The ions you have studied thus far have contained only one element.
                                        However, some ions contain more than one element. An ion that has two
                                        or more different elements is called a polyatomic ion. In a polyatomic
                                        ion, a group of atoms is covalently bonded together when the atoms share
                                        electrons. Although the individual atoms have no charge, the group as a
                                        whole has an overall charge. The formulas and names of some common
                                        polyatomic ions are shown in Table 5.2. Although the charge is shown to
                                        the right of the formula, it is the whole ion, rather than just the last atom
                                        listed, that is charged. Figure 5.5 shows models of three common poly-
                                        atomic ions.

158             Chapter 5     Types of Compounds
  Table 5.2 Common Polyatomic Ions

  Name of Ion                        Formula            Charge
  ammonium                             NH4                1
  hydronium                            H3O                1
  hydrogen carbonate                  HCO3                1
  hydrogen sulfate                    HSO4                1
  acetate                            C2H3O2               1
  nitrite                              NO2                1
  nitrate                              NO3                1
  cyanide                               CN                1
  hydroxide                            OH                 1
  dihydrogen phosphate                H2PO4               1
  permanganate                        MnO4                1
  carbonate                            CO32               2
  sulfate                              SO42               2
  sulfite                              SO32               2
  oxalate                             C2O42               2
  monohydrogen phosphate              HPO42               2
  dichromate                          Cr2O72              2
  phosphate                            PO43               3




                                           2                          2         Figure 5.5
                                                                                Polyatomic Ions
                                                                                Polyatomic ions, such as
                                                                                hydroxide (left), sulfate (cen-
                                                                                ter), and oxalate (right), are
                                                                                composed of more than one
                                                                                atom. Electrons are shared
 Hydroxide                       Sulfate                  Oxalate               between the atoms within the
   OH                            SO42                     C2O42                 ion, forming covalent bonds,
                                                                                but the ion as a whole has a
                                                                                charge. Thus, polyatomic ions
   Ionic compounds may contain positive metal ions bonded to negative
                                                                                form ionic bonds with other
polyatomic ions, such as in NaOH; negative nonmetal ions bonded to              ions to produce ionic com-
positive polyatomic ions, such as in NH4I; or positive polyatomic ions          pounds.
bonded to negative polyatomic ions, such as in NH4NO3. To write the for-
mula for an ionic compound containing one or more polyatomic ions,
simply treat the polyatomic ion as if it were a single-element ion by keep-
ing it together as a unit. Remember that the sum of the positive and nega-
tive charges must equal zero.
   Multiples of a polyatomic ion in a formula can be indicated by placing
the entire polyatomic ion, without the charge, in parentheses. Write a sub-
script outside the parentheses to show the number of polyatomic ions in
the compound. Never change the subscripts within the polyatomic ion. To
do so would change the composition of the ion. The formula for the com-
pound that contains one magnesium ion and two nitrate ions is Mg(NO3)2.

                                                                              5.1   Ionic Compounds        159
                                              Chemistry
Hard Water                                                cium ion or magnesium ion in the water is
                                                          exchanged for two sodium ions. Thus, the water
   The term hard water doesn’t describe water’s           leaving the ion exchanger has fewer calcium and
physical state. It describes water in which calcium,      magnesium ions but many more sodium ions.
magnesium, and hydrogen carbonate ions are dis-
solved. It is difficult to lather soap in hard water.
   One of the compounds in soap that helps pro-
duce lather is sodium stearate, NaC18H35O2, which
dissolves in water. In hard water, calcium ions
react with the stearate ions to form calcium
stearate, Ca(C18H35O2)2. This material is insoluble
and forms soap scum. Scum is often seen as a ring
around sinks or tubs. If calcium ions are removed
from the water by water softeners, soap will lather
and no more scum will form.
   Hard water can cause problems. When hard
water is heated, calcium carbonate is formed from
the reaction of calcium ions and hydrogen car-                            Exploring Further
bonate ions in the water. Because calcium carbon-
ate is not soluble in water, it forms thick scales        1. Interpreting What is the charge of the stearate
inside water heaters and water pipes. These scales           ion?
often clog pipes and keep the heater from properly        2. Thinking Critically Why do two sodium ions
heating the water.                                           replace one calcium or one magnesium ion?
Ion Exchangers A common way of softening                  3. Acquiring Information Why are detergents
water—that is, reducing the number of calcium                more effective than soaps in hard water?
and magnesium ions—is by an ion exchanger. The
ion exchanger usually contains a material, called a
resin, made up of carbon, hydrogen, and sodium              For more details about how water can be harder
ions. As hard water passes through the resin, a cal-        or softer, visit the Chemistry Web site at
                                                            chemistryca.com



                                      To name a compound containing a polyatomic ion, follow the same
                                   rules as used in naming binary compounds. Name the positive ion first,
                                   followed by the negative ion. However, do not change the ending of the
                                   negative polyatomic ion name. The name of the compound composed of
                                   calcium and the carbonate ion is calcium carbonate. Acids in groundwater
                                   can dissolve rocks made of calcium carbonate, such as limestone. Large,
                                   underground caverns are formed when the limestone is dissolved away
                                   slowly. Stalactites hanging from the ceiling and stalagmites rising from the
                                   ground are made when calcium carbonate precipitates from a water solu-
                                   tion dripping through cracks in the cavern ceiling.

160          Chapter 5   Types of Compounds
   What is the formula for calcium carbonate? Calcium is in Group 2, so
its ion has a 2 charge. The carbonate ion has a 2 charge, as shown in
Table 5.2. To form a neutral compound, one Ca2 ion must combine with
one CO32 ion to give the formula CaCO3.




 SAMPLE PROBLEM              2        Writing a Formula Containing a Polyatomic Ion

                                       Write the formula for the compound that contains lithium and
                                     carbonate ions.
                   Analyze       • Lithiumcarbonate ion1,has its2ioncharge,1andcharge. According to Table
                                   5.2, the
                                            is in Group so
                                                              a
                                                                     has a
                                                                                its structure is CO .3
                                                                                                      2

                    Set Up       • Write the symbols for lithium carbonate in formula form.
                                                              Li CO32
                     Solve       • Determine charges. In this case,lithium ions thecarbonateand negative
                                   ining their
                                               the correct ratio of
                                                                     the sum of
                                                                                to
                                                                                    positive
                                                                                             ions by exam-

                                     charges does not equal zero. Two lithium ions are needed to balance
                                     the carbonate ion. Because you cannot change the charges of the ions,
                                     you must add a subscript of 2 to Li . The correct formula for lithium
                                     carbonate is Li2CO3.
                    Check        •   Check to be sure that the overall charge of the formula is zero.
                                         2(1 ) (2 ) 0 The formula as written is correct.

 SAMPLE PROBLEM              3        Writing a More Complex Formula

                                       Write the formula for the compound that contains aluminum and
                                     sulfate ions.
                   Analyze       • Aluminumto Table 5.2, 13 and has ion oxidation charge. of 3 .
                                   According
                                               is in Group
                                                            the sulfate
                                                                        an
                                                                           has a 2
                                                                                    number

                    Set Up       • Write the symbols for aluminum sulfate in formula form.
                                                                3        2
                                                               Al SO 4

                     Solve       • Determine charges. In this case,aluminumofions to sulfateand negative
                                   ining their
                                               the correct ratio of
                                                                     the sum the positive
                                                                                             ions by exam-

                                     charges does not equal zero. To achieve neutrality, you must find the
                                     least common multiple of 3 and 2. The least common multiple is 6.
                                     How many Al3 ions will be needed to make a charge of 6 , and how
                                     many SO42 ions will be needed to make a charge of 6 ? It will be
                                     necessary to have two Al3 ions in the compound to balance three
                                     SO42 ions. You should add a subscript of 2 to the aluminum ion and a
                                     subscript of 3 to the sulfate ion. The entire polyatomic ion must be
                                     placed in parentheses to indicate that three sulfate ions are present.
                                     Thus, the correct formula for aluminum sulfate is Al2(SO4)3.
                    Check        •   Check to be sure that the overall charge of the formula is zero.
                                        2(3 ) 3(2 ) 0 The formula as written is correct.

                                                                             5.1   Ionic Compounds        161
  PRACTICE PROBLEMS

                                          3. Write the formula for the compound made from each of the follow-
                                             ing ions.
                                             a) ammonium and sulfite ions
For more practice with solving               b) calcium and monohydrogen phosphate ions
problems, see Supplemental
Practice Problems,
                                             c) ammonium and dichromate ions
Appendix B.                                  d) barium and nitrate ions
                                          4. Write the formula for each of the following compounds.
                                             a) sodium phosphate
                                             b) magnesium hydroxide
                                             c) ammonium phosphate
                                             d) potassium dichromate


Figure 5.6                            Compounds of Transition Elements
Two Compounds of Iron                    In Chapter 3, you learned that the elements known as transition ele-
and Sulfate                           ments are located in Groups 3 through 12 in the periodic table. Transition
Iron forms both Fe2 and Fe3           elements form positive ions just as other metals do, but most transition
ions, each of which can com-          elements can form more than one type of positive ion. In other words,
bine with the sulfate ion.            transition elements can have more than one oxidation number. For exam-
Some people use the older
                                      ple, copper can form both Cu and Cu2 ions, and iron can form both
spelling, sulphate.
                                      Fe2 and Fe3 ions. Figure 5.6 shows the two compounds that iron forms
                                      with the sulfate ion. Zinc and silver are two exceptions to the variability of
                                      other transition elements; each forms one type of ion. The zinc ion is
                                      Zn2 and the silver ion is Ag .

                                         Iron(II) sulfate, FeSO4 ,
                                         (sample on bottom left)
                                         is a blue-green crystalline      Iron(III) sulfate, Fe2(SO4)3, (sample on
                                         substance that is used in        right) is a yellow crystalline substance that
                                         fertilizer and as a food         is used as a coagulant in water-purification
                                         supplement.                      and sewage-treatment plants. After the
                                                                          sewage is coagulated, it is filtered out, as
                                                                          shown here.




162             Chapter 5   Types of Compounds
              ART            CONNECTION

China’s Porcelain
   See how proudly the dragon, an ancient symbol of Chi-
nese culture, prances around the vase pictured here.
Proud it should be, because this vase represents one of
the greatest achievements of Chinese technology and
art—glazed porcelain.
Clay, glaze, and fire By the third to sixth century
A.D., the Chinese had invented glazed porcelain. They
found that if a clay vessel, such as a bowl, is covered
with a transparent glaze and then heated to a high
temperature, a translucent ceramic material forms.
This material is glazed porcelain. Unlike a fired-clay
vessel, which remains slightly porous and opaque, the
translucent vessel is sealed by a glasslike covering. By
changing the chemical composition of the glaze, Chi-
nese artisans were able to change the quality and color of
the glaze. For example, when they added materials that
reacted with each other to form tiny gas bubbles in the
glaze, the porcelain appeared brighter because the surface
of the bubbles reflected light.
Colored glazes One of the most important steps in glazing
pottery was the addition of materials to the glaze to produce
colored porcelains. These materials were solutions of transi-
tion element ions, such as iron, manganese, chromium, cobalt,
copper, and titanium. During the firing of the glaze, these metals
formed oxides. Because the metal ions in the oxides reflected only certain wavelengths of light,
                          the glazes colored the porcelain. By varying the concentration and
                               charges of the metal ions in the glaze, the Chinese were able to
                                   produce subtly colored porcelains. For example, cobalt pro-
                                      duces a blue glaze, chromium a pink or green glaze
                                       depending on charge, and manganese a purple glaze.
                                         These beautiful colors have remained vivid over the
                                          course of thousands of years, and the techniques are
                                           still being used today.


                                                             Connecting to Chemistry
                                                 1. Applying Why are         2. Thinking Critically
                                                    porcelain dishes            What properties of
                                                    superior to wooden          metallic compounds
                                                    dishes?                     make them useful as
                                                                                colored glazes?


                                                                       5.1   Ionic Compounds       163
                               Chemists must have a way to distinguish the names of compounds
                            formed from the different ions of a transition element. They do this by
                            using a Roman numeral to indicate the oxidation number of a transition
                            element ion. This Roman numeral is placed in parentheses after the name
                            of the element. No additional naming system is needed for zinc and silver
                            compounds because their formulas are not ambiguous. Table 5.3 shows
                            the naming of the two different ionic compounds formed when chloride
                            ions combine with each of the two copper ions.


                               Table 5.3 Compounds of Copper and Chlorine

                               Copper Ion     Chloride Ion(s)     Formula            Name
                                  Cu                Cl              CuCl        copper(I) chloride
                                  Cu2              2Cl              CuCl2       copper(II) chloride




                            Table 5.4 shows the chemical names of some transition element ions.
                            When you do Practice Problems 5 and 6, you will become familiar with
                            these names. Note in the photos accompanying the table that the different
                            ions of a transition element often form compounds of different colors.
                            For example, CrO is black, Cr2O3 is green, and CrO3 is red. Determine the
                            oxidation number for chromium in each of these compounds.


                               Table 5.4 Names of Common Ions of Selected Transition Elements

                               Element              Ion            Chemical Name
                               Chromium             Cr2            chromium(II)
                                                    Cr3            chromium(III)
                                                    Cr 6           chromium(VI)
                               Cobalt               Co2            cobalt(II)                 CrO
                                                    Co3            cobalt(III)
                               Copper               Cu             copper(I)
                                                    Cu2            copper(II)
                               Gold                 Au             gold(I)
                                                    Au3            gold(III)
                                                                                                 Cr2O3
                               Iron                 Fe2            iron(II)
                                                    Fe3            iron(III)
                               Manganese            Mn2            manganese(II)
                                                    Mn3            manganese(III)
                                                    Mn7            manganese(VII)
                               Mercury              Hg             mercury(I)
                                                    Hg2            mercury(II)
                               Nickel               Ni2            nickel(II)
                                                    Ni3            nickel(III)                 CrO3
                                                    Ni4            nickel(IV)


164   Chapter 5   Types of Compounds
   Suppose you wanted to write the formula for a compound containing
a transition element. Look back at Sample Problem 1, where you learned                          Lab
to write the formula for a compound containing sodium and sulfur.
How would you write the formula if it were iron(II) rather than sodium                 See page 865 in
that combined with sulfur? Iron(II) has an oxidation number of 2 ,                     Appendix F for
and its ion can be written as Fe2 . You know that the sulfide ion has a                   Iron Ink
charge of 2 and can be written as S2 . The charges balance in this case,
and the formula for iron(II) sulfide is written as FeS, Figure 5.7.
   You can write the formula for iron(III) sulfide in the same way. Just
follow the steps in Sample Problem 3. The correct formula for iron(III)
sulfide is Fe2S3. Note that the Roman numeral refers to the oxidation
number of the iron and not to how many ions are in the formula.
   How can you name a compound of a transition element if you are
given the formula? Determining the charge of the transition element
ion gives the clue needed to name the compound. In the formula
Cr(NO3)3, you must determine the charge of the chromium ion in order
to name the compound. Look first at the negative ion. Knowing that the
nitrate ion has a charge of 1 and that there are three nitrate ions with
a total charge of 3 , you can see that the chromium ion must have a           Figure 5.7
charge of 3 to maintain neutrality. Thus, this compound is named              Fool’s Gold
chromium(III) nitrate.                                                        Iron disulfide (FeS2 ), is com-
                                                                              monly called fools gold. In this
                                                                              ionic compound, sulfur exists
                                                                              in the unusual form S22–, and
                                                                              iron has an oxidation number
                                                                              of 2+.

  PRACTICE PROBLEMS

                                 5. Write the formula for the compound made from each of the
                                    following pairs of ions.
                                    a) copper(I) and sulfite
For more practice with solving      b) tin(IV) and fluoride
problems, see Supplemental
Practice Problems,
                                    c) gold(III) and cyanide
Appendix B.                         d) lead(II) and sulfide
                                 6. Write the names of the following compounds.
                                    a) Pb(NO3)2
                                    b) Mn2O3
                                    c) Ni(C2H3O2)2
                                    d) HgF2


Hydrates
   Many ionic compounds are prepared by crystallization from a water
solution, and water molecules become a part of the crystal. A compound
in which there is a specific ratio of water to ionic compound is called a
hydrate. In a hydrate, the water molecules are chemically bonded to the
ionic compound.

                                                                            5.1   Ionic Compounds         165
                                    A Chemical Weather Predictor
                                      Adding water to an anhydrous compound to form a hydrate often
                                    changes the physical properties—such as color—of the compound.

                  1                 Cobalt(II) chloride is such a compound. If you find that the color of
                                    the compound changes in accordance with the weather, perhaps
                                    cobalt(II) chloride can serve as a weather predictor.
                                    Procedure
                                                                           5. Keep your weather predictor in
                                                                              a convenient location, and
                                           1. Place 5 mL of 95 percent        check its color each morning
                                              ethanol in a small beaker.      and afternoon. Keep a three-
                                           2. Use a spatula to add a          week log of the time, the cur-
                                              small amount of                 rent weather, and the color of
                                              cobalt(II) chloride to the      the treated paper.
                                              beaker. Stir until the       Analysis
                                              compound dissolves.          1. What is the formula of
                                           3. Dip a cotton swab into          cobalt(II) chloride?
                                              the pink solution and use    2. The hydrate of cobalt(II) chlo-
                                              it to write the chemical        ride has six water molecules
                                              formula of cobalt(II)           bonded to it. What is its for-
                                              chloride on a piece of          mula?
                                              white paper.                 3. From your observations, are
                                           4. Dry the paper by holding        you able to conclude that the
                                              it over a hot plate set on      cobalt(II) chloride test paper is
                                              low or by putting it in a       a reliable weather predictor?
                                              sunny location. What            Justify your answer.
                                              color is the formula now?


                                       Does your chemistry instructor often remind students to make sure that
                                    the lids on jars of chemicals are tightly closed? There is a good reason for
                                    sealing the jars tightly; some ionic compounds can easily become hydrates
                                    by absorbing water molecules from water vapor in the air. These com-
                                    pounds are called hygroscopic substances, and one example is sodium car-
                                    bonate (Na2CO3). Substances that are so hygroscopic that they take up
                                    enough water from the air to dissolve completely and form a liquid solution
                                    are called deliquescent. A deliquescent substance is shown in Figure 5.8.


Figure 5.8
Deliquescent Substances
Sodium hydroxide (left) is an example
of a deliquescent substance because it
has a strong attraction for water mol-
ecules. Sodium hydroxide will absorb
water molecules from the surround-
ing air and begin to dissolve (right).
Eventually, it will absorb enough
water to dissolve completely.

166           Chapter 5   Types of Compounds
Cement
   People have been using cementing materials for thousands
of years. The stones in the Egyptian pyramids are held togeth-
er by a mixture of sand and the mineral compound gypsum,
which is calcium sulfate dihydrate. When this dihydrate is
heated, water evaporates, forming a compound with one water
molecule per two calcium sulfate formula units. Today, we
know this binding material
as plaster of paris.



1. Cement is made
   from a mixture of                                                4. Cement can be used alone to
   limestone and clay.                                                  form a smooth, hard surface for
   The most important                                                   roads or buildings, or it can be
   minerals in clay are                                                 combined with sand and gravel
   the combinations of                                                  to form a rougher material called
   aluminum,                                                            concrete.
   oxygen, and
   silicon                                                          5. When concrete is mixed with
   known as                                                             water, silicate compounds
   aluminum                                                             hydrate and form gelatinous
   silicates.                                                           materials called gels.

                                                                    6. The hardening process takes sev-
                                                                        eral days. During this time, some
                                                                        water is removed from the gels
                                                                        that formed around the sand and
                                                                        gravel, and calcium hydroxide
                                                                        absorbs carbon dioxide from the
                                                                        air to re-form calcium carbonates.
                                                                        Fibers that form from the cement
                                                                        materials interlock and strength-
                                                                        en the concrete.


2. Before this limestone-clay mixture can be
   used, it must be heated. Heating drives off                    Thinking Critically
   carbon dioxide and forms new ionic com-
   pounds. This new mixture of calcium silicates,    1. What is the formula        hydrated during
   calcium aluminates, and calcium aluminum             for the calcium sul-       solidification of
   ferrates forms in clumps called clinker.
                                                        fate dihydrate that        cement to form
3. The clinker is ground and mixed with small           makes up gypsum?           Ca3Al2O6 6H2O.
   amounts of calcium sulfate. This mixture is       2. Tricalcium aluminate       What is the name of
   called portland cement.                              also becomes               this hydrate?

                                                                         5.1   Ionic Compounds        167
        Figure 5.9
        Desiccants
        Boxes of electronic equipment such as cam-
        eras and CD players usually contain small
        packets of a desiccant. The desiccant absorbs
        water vapor from the air, protecting the deli-
        cate metal parts against corrosion and pre-
        venting condensation of water vapor in the
        wiring of the equipment. Even a tiny quanti-
        ty of water on a circuit board can create a
        short circuit. Desiccants are especially useful
        for packaging electronic equipment that is to
        be shipped overseas because ocean air in the
        holds of ships contains so much moisture.




                                Many of these compounds become hydrates by absorbing water from
                            the air. As shown in Figure 5.9, compounds that form hydrates often are
                            used as drying agents, or desiccants, because they absorb so much water
                            from the air when they become hydrated.
                                To write the formula for a hydrate, write the formula for the compound
                            and then place a dot followed by the number of water molecules per for-
                                                 mula unit of compound. The dot in the formula repre-
                                                 sents a ratio of compound formula units to water mol-
  Table 5.5 Prefixes to Use in                   ecules. For example, CaSO4 2H2O is the formula for a
             Naming Hydrates                     hydrate of calcium sulfate that contains two molecules
                                                 of water for each formula unit of calcium sulfate. This
  Molecules of Water       Prefix                hydrate is used to make portland cement and plaster of
           1                mono-                paris. To name hydrates, follow the regular name for
           2                  di-                the compound with the word hydrate, to which a pre-
           3                  tri-               fix has been added to indicate the number of water
           4                tetra-               molecules present. Use Table 5.5 to find the correct
           5                penta-               prefix to use. The name of the compound with the for-
           6                hexa-                mula CaSO4 2H2O is calcium sulfate dihydrate.
           7                hepta-                  Heating hydrates can drive off the water. This results
           8                 octa-               in the formation of an anhydrous compound—one in
           9                nona-                which all of the water has been removed. In some
          10                deca-                cases, an anhydrous compound may have a different
                                                 color from that of its hydrate, as shown in Figure 5.10.


                                   Interpreting Formulas
                                     You have learned how to write a formula to represent a formula unit of
                                   an ionic compound. Sometimes, it may be necessary to represent more
                                   than one formula unit of a compound. To do this, place a coefficient
                                   before the formula. Two formula units of NaCl are represented by 2NaCl,
                                   three formula units by 3NaCl, and so on.

168          Chapter 5   Types of Compounds
   A formula summarizes how many atoms of each element are present.          Figure 5.10
Each formula unit of sodium chloride contains one sodium ion and one         Forming an Anhydrous
chloride ion. How many oxygen atoms are present in 3HNO3? Each for-          Compound
mula unit contains three oxygen atoms. Because there are three formula       When blue copper(II) sulfate
units, a total of nine atoms of oxygen are present. As another example,      pentahydrate (CuSO4 5H2O)
consider how many atoms of hydrogen are in one formula unit of ammo-         (left) is heated, the water is
nium sulfate. The formula for ammonium sulfate is (NH4)2SO4. Each            driven off (center). The anhy-
ammonium ion contains four atoms of hydrogen. Because two ammoni-            drous compound, CuSO4
                                                                             (right), is white. Hydrated
um ions are present, there are eight atoms of hydrogen in a formula unit
                                                                             copper sulfate is used as a
of ammonium sulfate. How many hydrogen atoms are in 3(NH4)2SO4?              fungicide in water reservoirs.
To find out, simply multiply the eight hydrogen atoms in one formula
unit by three formula units; 24 hydrogen atoms are present.



                                      SECTION REVIEW
Understanding Concepts                                 d) Na2CO3
1. Explain why ionic compounds cannot conduct          e) KMnO4
   electricity when they are in the solid state.       f) Ni(OH)2
                                                       g) NaC2H3O2
2. Write formulas for each of the following ionic
   compounds.                                       Thinking Critically
   a) manganese(II) carbonate                       4. Interpreting Chemical Formulas What infor-
   b) barium iodide dihydrate                          mation does the formula 3Ni(HCO3)2 tell you
   c) aluminum oxide                                   about the number of atoms of each element
   d) magnesium sulfite                                that are present?
   e) ammonium nitrate
                                                    Applying Chemistry
   f) sodium cyanide
                                                    5. Toothpaste Ingredients Examine the ingredi-
3. Name the ionic compound represented by each         ent label on a tube of toothpaste. Write formu-
   formula.                                            las for as many of the chemical names listed as
   a) Na2SO4                                           you can. List whether each ingredient is an
   b) CaF2                                             ionic or a covalent compound.
   c) MgBr2 6H2O


            chemistryca.com/self_check_quiz                                5.1   Ionic Compounds        169
            SECTION

           5.2                      Molecular Substances
                                    H
                                           ow many compounds can you name that are liquids or gases at normal
 SECTION PREVIEW                           room temperature? Water, carbon dioxide, and ammonia are just a few
                                           examples. Because most ionic compounds are solids at room tempera-
 Objectives                         ture, the odds are pretty good that any compounds you thought of are mem-
                                    bers of the other major class of compounds described in Chapter 4 —the
 ✓ Compare the prop-
                                    covalent compounds. However, not all of them are liquids or gases. Some
 erties of molecular
 and ionic substances.              covalent compounds are solids at room temperature, for example sugar,
                                    mothballs, silica (sand), and
 ✓ Distinguish among
                                    the fats that make up butter
 allotropes of an
 element.                           and margarine. Most of the
                                    time, it is difficult to tell
 ✓ Apply formulas to
                                    whether a solid compound is
 name molecular
 compounds.                         ionic or covalent by visual
                                    examination alone. Compare
 Review Vocabulary                  the crystals of sugar and salt
 Anhydrous: a com-                  shown here.
 pound in which all of
 the water has been
 removed, usually by                Properties of Molecular Substances
 heating.
                                       You know that ionic compounds share many properties. The properties
 New Vocabulary                     of a molecular substance—a substance that has atoms held together by
 molecular substance                covalent rather than ionic bonds—are more variable than the properties
 distillation                       of ionic compounds. Some molecular substances, such as polyethylene
 molecular element                  plastic and the fats in butter, are soft; rubber is elastic; and diamond and
 allotrope                          quartz are hard.
 organic compound                      Although molecular substances have varied properties, some generalities
 inorganic compound                 can be made to distinguish them from ionic compounds. Molecular sub-
 hydrocarbon                        stances usually have lower melting points, and most are not as hard as ionic
                                    compounds, Figure 5.11. In addition, most molecular substances are less
                                    soluble in water than ionic compounds and are not electrolytes. The prop-
                                    erties of most ionic and molecular substances are different enough that
                                    their differences can be used to classify and separate them from one


                                                                          Figure 5.11
                                                                          Crayons—Covalent or Ionic?
                                                                          Crayons are made of covalent com-
                                                                          pounds. They are soft and are insoluble
                                                                          in water. If you have ever left crayons
                                                                          out in the sun, you know that they also
                                                                             have a low melting point.

170           Chapter 5   Types of Compounds
  Where’s the calcium?
     Calcium is an important part of the structure of bones and eggshells.
  If a bone is soaked in vinegar for several days, the structure of the bone
  will change. Vinegar contains acetic acid, which reacts with the calcium
  compounds in the bone to form calcium acetate.                                                             2
  Procedure
                                             present in the vinegar, it will
                                             give an orange-red flame test.
  1. Pick most of the meat from a        Analysis
     small, uncooked chicken bone.       1. Describe the change in the
  2. Place the bone in a beaker,            properties of the bone after two
     cover it with vinegar, and cover       days and after four days.
     the beaker with a watch glass.
                                         2. If the flame test verified the
  3. Label the beaker with your name        presence of calcium ions in the
     and leave it for two days in the       vinegar, what was the probable
     area indicated by your teacher.        source of the calcium?
  4. Use forceps to remove the bone      3. What do you conclude regard-
     from the beaker, and blot it on a      ing the effect of ionic calcium
     paper towel. Examine the bone          compounds on the properties
     and observe how it has changed.        of bone? Do their properties
  5. Replace the bone in the vinegar        in bone seem to correlate
     and let it soak for two more           with those of typical ionic
     days. Repeat step 4.                   compounds?
                                                                                   Calcium compounds make
  6. Straighten a paper clip. Hold-      4. How do the properties of the           bones hard but brittle. Apply-
     ing the clip with forceps, dip it      bone after soaking reflect the         ing stress to brittle materials
     into the vinegar solution, then        presence mainly of covalent            can cause them to break.
     hold it in the blue flame of a         compounds in the bone?
     Bunsen burner. If calcium is


another. The separation of water from salt by distillation is one example
that makes use of these property differences. Distillation is the method of
separating substances in a mixture by evaporation of a liquid and subse-
quent condensation of its vapor. As you learned, solar stills make use of this
method. A simple lab-distillation apparatus is shown in Figure 5.12.               Figure 5.12
                                                                                   Distillation in the Lab
   100°C               Cool water out                                              A soluble ionic compound
                                                                                   such as NaCl can be separated
                   Steam                                                           from water using a distillation
                                                      Condenser                    apparatus like this one. As the
                                                                                   salt water is boiled in the dis-
                     Distillation                                                  tillation flask, the water turns
                     flask                                                         to steam and the salt is left
                                                                    Receiving
                                                                                   behind. The steam passes
                     Salt water                                     flask
                                                                                   through a water-cooled con-
                                                  Cold                Distilled    denser, where it condenses
                                                  water in            water        into pure, distilled water. The
                                                                                   distilled water is collected in
                                                                                   the receiving flask.

                                                                            5.2   Molecular Substances           171
                                                                                                 SMALL
                                                                                                 SCALE




  Ionic or Covalent?                                        graduated cylinder, small
                                                            thermometer (must read up to 150°C)
     Compounds can be classified by the types of            1- to 2-g samples of any 4 of the following:
  bonds that hold their atoms together. Ions are               salt substitute (KCl), fructose, aspirin, paraffin,
  held together by ionic bonds in ionic com-                   urea, table salt, table sugar, Epsom salt
  pounds; atoms are held together by covalent
  bonds in molecular compounds.                             Safety Precautions
     You cannot tell whether a compound is ionic or           Use care when handling hot objects.
  molecular simply by looking at a sample of it
  because both types of compounds can look similar.
  However, simple tests can be done to classify com-
  pounds by type because each type has a set of char-
  acteristic properties shared by most members. Ionic        1. Use a grease pencil or crayon to draw lines
  compounds are usually hard, brittle, water-soluble,           dividing a glass slide into four parts. Label the
  have high melting points, and can conduct electric-           parts A, B, C, and D.
  ity when dissolved in water. Molecular compounds           2. Make a data table similar to the one shown in
  can be soft, hard, or flexible; are usually less water-       Data and Observations.
  soluble; have lower melting points; and cannot             3. Use a spatula to place about one-tenth (about
  conduct electricity when dissolved in water.                  0.1 to 0.2 g) of the first of your four sub-
  Problem                                                       stances on section A of the slide.
    How can you identify ionic and molecular                 4. Repeat step 3 with your other three substances
  compounds by their properties?                                on sections B, C, and D. Be sure to use a clean
                                                                spatula for each sample. Record in your data
  Objectives
                                                                table which substance was put on each section.
  •Examine the properties of several common
   substances.                                               5. Place the slide on a hot plate. Turn the heat
                                                                setting to medium and begin to heat the slide.
  •Interpret the property data to classify each sub-
   stance as ionic or molecular.                             6. Gently hold a thermometer so that the bulb
                                                                just rests on the slide. Be careful not to dis-
                                                                turb your compounds.


  Materials                                                     Melting point
  glass microscope slide                                        determination
  grease pencil or crayon
  hot plate
  spatula                                                                        A   B   C   D

  4 small beakers (50- or 100-mL)
  stirring rod
  balance
  conductivity tester



172          Chapter 5   Types of Compounds
 7. Continue heating until the temperature
    reaches 135°C. Observe each section on the
    slide and record which substances have melt-
    ed. Turn off the hot plate.                        1. Interpreting Observations What happened
                                                          to the bonds between the molecules when a
 8. Label four beakers with the names of your             substance melted?
    four substances.
                                                       2. Comparing and Contrasting Did all com-
 9. Weigh equal amounts of the four substances            pounds melt at the same temperature?
    (1-2 g of each), and place the weighed sam-
    ples in their labeled beakers.                     3. Classifying Complete your data table by
                                                          classifying each of the substances you tested as
10. Add 10 mL of distilled water to each beaker.          ionic or molecular compounds based on your
11. Stir each substance, using a clean stirring rod       observations.
    for each sample. Note on your table whether
    or not the sample dissolved completely.
12. Test each substance for the presence of elec-
    trolytes by using a conductivity tester. Record
    whether or not each acts as a conductor.           1. What are the differences in properties between
                                                          ionic and molecular compounds?
                                                       2. How did the melting points of the ionic com-
                                                          pounds and the molecular compounds com-
                                                          pare? What factors affect melting point?
                                                       3. The solutions of some molecular compounds
                                                          are good conductors of electricity. Explain
                                                          how this can be true when ions are required to
                                                          conduct electricity.
                                                       4. Consider a mixture of sand, salt, and water.
                                                          How can you make use of the differences in
                                                          properties of these materials to separate them?




                                              Did it dissolve       Did the solution
      Substance           Did it melt?          in water?         conduct electricity?        Classification

          A

          B

          C

          D




                                                                           5.2   Molecular Substances          173
                                       How does the submicroscopic structure of molecular substances con-
                                    tribute to their macroscopic properties? Because there are no ions, strong
                                    networks held together by the attractions of opposite charges do not
                                    form. The interparticle forces between molecules are often weak and easy
                                    to break. These weak forces explain the softness and low melting points of
                                    most molecular substances. Most molecular substances are not elec-
                                    trolytes because they do not easily form ions.

                                    Molecular Elements
Figure 5.13
                                       Molecules vary greatly in size. They can contain from just two to thou-
Size of Molecules
Molecular substances can be
                                    sands or millions of atoms, as Figure 5.13 shows. Most elements usually
as simple as two iodine atoms       occur naturally in a combined form with another element; that is, they
linked together as I2 (bottom)      occur as compounds. However, in some cases, two or more atoms of the
or as complex as this protein,      same element can bond together to form a molecule. A molecule that
cytochrome c (top), which           forms when atoms of the same element bond together is called a molecu-
contains many thousands of          lar element. Note that molecular elements are not compounds—they
atoms of carbon, hydrogen,          contain atoms of only one element. Why do atoms of these elements bond
oxygen, nitrogen, and sulfur        so readily to identical atoms? When they bond together, each atom
linked together by covalent         achieves the stability of a noble-gas electron configuration.
bonds.




                                       Cytochrome c is found in all living cells that derive energy by break-
                                       ing down food molecules in the presence of oxygen, and it is found
                                       in especially large quantities in hard-working muscle tissue.


                                    Diatomic Elements
                                       Seven nonmetal elements are found naturally as molecular elements of
                                    two identical atoms. The elements whose natural state is diatomic are
      I2                            hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine. Their
                                    formulas can be written as H2, N2, O2, F2, Cl2, Br2, and I2, respectively.
                                    These molecules are referred to as diatomic elements. All except bromine
                                    and iodine are gases at room temperature; Br2 is a liquid, and I2 is a solid.

174           Chapter 5   Types of Compounds
   What can you learn by examining the structures of the diatomic ele-
ments? Electron dot diagrams offer clues. As an example, the chlorine
atom has seven valence electrons and needs one more to achieve the con-
figuration of the noble gas argon. If two chlorine atoms combine, they
share a single pair of electrons, and each atom attains a stable octet con-
figuration.
                                                                                          allotrope:
                         Cl    Cl ˇ Cl Cl                                                  allos (GK) other
Hydrogen, fluorine, bromine, and iodine molecules also are formed by the                   tropos (GK) way,
                                                                                           manner
sharing of a single pair of electrons. Two oxygen atoms share two pairs of
                                                                                          Allotropic elements
electrons to form O2, and two nitrogen atoms share three pairs of elec-
                                                                                          have other ways of
trons to form N2.                                                                         being arranged.
                           O O         N N

Allotropes
   Although the diatomic form of oxygen, O2, is most common in our
atmosphere, oxygen also exists as O3—ozone. The structure of ozone is
different from that of diatomic oxygen. It consists of three atoms of oxy-
gen rather than the two atoms in diatomic oxygen.                               Figure 5.14
                                                                                Uses of Ozone
                               O OO                                             Ozone produced in machines
Molecules of a single element that differ in crystalline or molecular struc-    like this is used to treat cloth-
ture are called allotropes. The properties of allotropes are usually differ-    ing, carpeting, and other
ent even though they contain the same element. This is because structure        materials that have been dam-
can be more important than composition in determining properties of             aged by smoke and soot from
                                                                                a fire. Hotels also use ozone
molecules.
                                                                                machines to remove the odor
   Oxygen and ozone are allotropes. The oxygen we breathe is O2; it is          of cigarette smoke from
found in the atmosphere. Ozone also occurs naturally and is formed from         rooms. The reactive ozone
diatomic oxygen by lightning or ultraviolet light. You may have smelled         oxidizes large, smelly com-
the sharp odor of ozone during an electrical storm as it formed in the          pounds in the smoke and soot
atmosphere by the action of lightning. Small amounts of ozone also are          into smaller, odorless com-
formed in TV sets or computer monitors when an electrical discharge             pounds.
passes through the oxygen in the
air. Have you ever smelled it when
sitting close to the screen?
Because ozone is harmful to liv-
ing things, it is advisable that you
not sit close to your TV set or
computer.
   Although ozone formed near
the surface of Earth is an undesir-
able component of smog, ozone
also has many uses, such as that
shown in Figure 5.14 and in puri-
fying water. The layer of ozone
found high in Earth’s atmosphere
is helpful because it shields living
things from harmful ultraviolet
radiation from the sun.

                                                                         5.2   Molecular Substances             175
                                    CHEMISTRY
               TECHNOLOGY   &
  Carbon Allotropes:
  From Soot to Diamonds
     Carbon is the most versatile element
  in forming allotropes. Organized or
  unorganized, atoms of carbon can
  take on an incredible number of
  arrangements, each different from the
  other and each forming a different allotrope.                                                        Graphite
  With all their diversity, these substances have one
  thing in common: they are made up solely of
  covalently bonded carbon atoms.

  Graphite
     The most familiar form of carbon is graphite.
  Mixed with a little clay and formed into a rod, it
  becomes the lead in a pencil. Look at the struc-
  ture of graphite. As you can see, the carbon          Carbon Blacks
  atoms are linked to each other in a continuous
  sheet of hexagons (six-sided figures). Note that         Carbon blacks make up most of the soot that
  each carbon atom connects three different hexa-       collects in chimneys and becomes a fire hazard.
  gons. It’s clear that the structure of graphite is    They are formed by the incomplete burning of
  well organized. The arrays of hexagons are            hydrocarbon compounds, as shown here. Each
  arranged in layers that are loosely held together.    microscopic chunk of a carbon black is made of
  The looseness between layers                          millions of jumbled chunks of layered carbon
  is why graphite is useful in                          atoms. However, the layers lack the organization
  pencil lead. As you write,                            of graphite, giving carbon black its haphazard
  the surface of the paper                                 structure. Carbon blacks are used in the pro-
  pulls off the loosely held                                duction of printing inks and rubber products.
  layers of carbon atoms.

                                                                                     Carbon black




176
Diamond
   Another allotrope of elemental carbon is dia-
mond. Besides being blinded by the brilliance of
a cut diamond, you should know that diamond is
the hardest natural substance. It’s often used on
the tips of cutting tools and drills. Can the struc-
ture of diamond explain its hardness? Look at the
model of diamond. Every carbon atom is
attached to four other carbon atoms which, in
turn, are each attached to four more carbon
atoms. Diamond is one of the most organized of
all substances. In fact, every diamond is one huge
molecule of carbon atoms. This organization of
covalently bonded carbons throughout diamond
accounts for its hardness. If you tried to write
with a diamond, you’d only tear your paper
because layers of carbon atoms do not slip off as
they do in graphite. The organization of carbon
atoms into diamond occurs under extreme pres-
sure and temperature, often at depths of 200 km                                                   Diamond
and over a long period of time. Diamonds range
in age from 600 million to 3 billion years old.




  Charcoals

                                                       Charcoals
                                                          Charcoals—the kind you draw with or cook
                                                       with—are another type of poorly organized car-
                                                       bon molecules. Charcoals are produced from the
                                                       burning of organic matter. If you look closely at a
                                                       chunk of charcoal, you can see that it’s extremely
                                                       porous. All these pores, pock marks, and holes
                                                       give charcoal a large surface area. Some charcoal,
                                                       called activated charcoal, has as much as 1000 m2
                                                       of surface area per gram. This property makes
                                                       activated charcoal useful in filtering water. Mole-
                                                       cules, atoms, and ions responsible for unwanted
                                                       odors and tastes in water are attracted to and held
                                                       by the surface of the activated charcoal as water
                                                       passes through it in this water-filtering pitcher.


                                                                          5.2   Molecular Substances        177
  Fullerenes                                              of a group of highly organized allotropes of car-
                                                          bon called fullerenes. Buckminsterfullerene was
    This is a model of buckminsterfullerene, C60,         discovered in soot in 1985, and its soccer-ball
  which was named after the engineer and architect        shape was confirmed in 1991. Since then, other
  Buckminster Fuller, who invented the geodesic                            naturally occurring and artificial-
  dome shown here. Both the dome and the mole-                                  ly produced fullerenes have
  cule are unusually stable. The molecule is one                                   been identified. Fullerenes
                                                                                     have even-numbered
                                                                                      molecular formulas
                                             Fullerenes
                                                                                       such as C70 and C78.
                                                                                       The molecules of some
                                                                                       fullerenes are hollow
                                                                                      spheres, whereas mole-
                                                                                    cules of others are hollow
                                                                                  tubes. The cagelike struc-
                                                                              tures of fullerenes are very
                                                                              flexible. After crashing into
                                                                              steel plates at speeds of
                                                                              7000 m/s (about 16 000
                                                                              miles/hour), C60 molecules
                                                                              rebound with their original
                                                                              shapes intact.



  Linear Acetylenic Carbon
     This threadlike allotrope of carbon is orga-
  nized into long spirals of bonded carbon atoms.
  Each spiral contains 300 to 500 carbon atoms. It’s
  produced by using a laser to zap a graphite rod in
  a glass container filled with argon gas. The
  allotrope splatters on the glass walls and is then
  removed. Because they conduct electricity, these
  carbon filaments may have uses in microelec-
  tronics. Some linear acetylenic carbons may
  eventually form fullerenes, whereas others form                        Linear acetylenic carbon
  soot.


                                      DISCUSSING THE TECHNOLOGY
  1. Applying From their structures, predict how             cule of linear acetylenic carbon? A molecule of
     buckminsterfullerene, diamond, and graphite             diamond?
     rank in increasing order of mass density.            3. Acquiring Information What might be some
     Explain.                                                possible uses of fullerenes and linear acetylenic
  2. Thinking Critically How would you describe              carbons?
     a molecule of buckminsterfullerene? A mole-


178         Chapter 5   Types of Compounds
   Phosphorus has three common allotropes: white, red, and black. All are        Figure 5.15
formed from P4 molecules that are joined in different ways, giving each          Phosphorus Allotropes
allotrope a unique structure and properties, as shown in Figure 5.15.            The white, red, and black
   Carbon has several important allotropes with different properties. Dia-       allotropes of phosphorus have
mond is a crystal in which the atoms of carbon are held rigidly in place in a    different properties. Note the
three-dimensional network. In graphite, the carbon atoms are held together       differences in their structures.
closely in flat layers that can slide over each other. This property makes
graphite soft and greasy-feeling and useful as a dry lubricant in locks.
   Another set of carbon allotropes, the fullerenes, consist of carbon atom
clusters. These molecules are unusually stable and are an exciting area of                          White
research for chemists because of their potential use as superconductors.                          phosphorus

Black phosphorus is a semiconductor,
whereas the other two forms are not.




                                                  Black                            Red
                                               phosphorus                       phosphorus


                                                                                     White phosphorus will
                                                                                     ignite spontaneously in air,
                                                                                     whereas the red form
                                                                                     won’t ignite unless it con-
                                                                                     tacts a flame. For these
                                                                                     reasons, white phosphorus
                                                                                     (left) must be stored under
                                                                                     water, and red phosphorus
                                                                                     (right) is used in the strike
                                                                                     pad of safety matches.




Formulas and Names of Molecular Compounds
   Molecular compounds make up a large group; millions of molecular
compounds are already known, and scientists are likely to discover or cre-
ate many others. How can you possibly begin to study so many com-
pounds? Before you can study their structures and properties and learn
how these properties determine their usefulness, you should be able to
name the compounds and write their formulas. Fortunately, chemists
have devised a naming system for molecular compounds that is based on
a much smaller number of rules than there are compounds.

                                                                          5.2   Molecular Substances           179
                                    Naming Binary Inorganic Compounds
                                       Substances are either organic or inorganic. Compounds that contain
                                    carbon, with a few exceptions, are classified as organic compounds. Com-
                                    pounds that do not contain carbon are called inorganic compounds.
                                    How are inorganic compounds held together? If inorganic compounds
                                    contain only two nonmetal elements, they are bonded covalently and are
                                    referred to as molecular binary compounds.
                                       To name these compounds, write out the name of the first nonmetal
                                    and follow it by the name of the second nonmetal with its ending
                                    changed to -ide. How do you know which element to write first? You write
                                    first the element that is farther to the left in the periodic table, with the
                                    exceptions of a few compounds that contain hydrogen. If both elements
                                    are in the same group, name first the element that is closer to the bottom
                                    of the periodic table. For example, sulfur dioxide is a compound contain-
                                    ing sulfur and oxygen. The sulfur is named first because it is closer to the
                                    bottom of the periodic table than oxygen is.
                                                                                 When naming a binary ionic
Figure 5.16                                                                   compound, this is the last step.
Carbon Disulfide                                                              However, because nonmetal atoms
The compound represented
                                                                              can share different numbers of elec-
by the formula CS2 is named
                                                                              tron pairs, several different com-
carbon disulfide because two
sulfur atoms are bonded to                                                    pounds can be formed from the
one carbon.                                                                   same two nonmetal elements. Thus,
                                    an additional step is necessary to give an unambiguous name to a mole-
                                    cule. To name the compound correctly, add a prefix to the name of each
                                    element to indicate how many atoms of each element are present in the
                                    compound. The same prefixes that were used to indicate the number of
                                    water molecules in hydrates are used here. For example, CS2 is named car-
                                    bon disulfide, Figure 5.16. Refer to Table 5.5 to review these prefixes.
                                       A few other rules are helpful when naming molecular compounds. If
                                    only one atom of the first element is listed, the prefix mono is usually
                                    omitted. Also, if the vowel combinations o-o or a-o appear next to each
                                    other in the name, the first of the pair is omitted to simplify pronuncia-
                                    tion. Thus, mononitrogen monooxide, NO, becomes nitrogen monoxide.
                                       Now you are ready to practice naming molecular compounds. Several
                                    different molecules can be formed when different numbers of nitrogen
                                    and oxygen atoms combine. Look at their formulas in the first column of
                                    Table 5.6, and try to name them without looking at the names listed in
                                    the second column. The brown gas pictured is NO2.


                                       Table 5.6 Formulas and Names of Some Molecular Compounds

                                      Formula                 Name
                                        NO             nitrogen monoxide
                                        NO2            nitrogen dioxide
                                        N2O            dinitrogen monoxide
                                        N2O5           dinitrogen pentoxide


180           Chapter 5   Types of Compounds
   Consider the two compounds that contain carbon and oxygen. The car-
bon contained in wood is converted to carbon dioxide when wood burns
completely. The formula for this product is CO2. If the carbon in wood
burns incompletely, the highly toxic gas carbon monoxide is formed.
                                                                                      N2O3
What is the formula for carbon monoxide?                                        dinitrogen trioxide
   To write the formula of a molecular compound for which you are given
the name, first write the symbols of each element in the order given in the    Figure 5.17
name. Then add the appropriate subscript after each element that has two       Formulas of Binary
or more atoms present. Remember that the prefixes in the name tell how         Molecules
many atoms of each element are present. For example, the compound sul-         The formula for dinitrogen tri-
fur hexafluoride contains the elements sulfur and fluorine. Because the        oxide is written N2O3. Analyze
word sulfur has no prefix, it is understood that there is only one sulfur      the name of this compound
atom; thus, the symbol S does not require a subscript. The prefix hexa         to determine how its formula
tells you that six fluorine atoms are in the compound, so the subscript 6      is written.
must be added to the F. The formula for sulfur hexafluoride is SF6. Follow
the rules for writing a formula for a molecular compound as you examine
the formula shown in Figure 5.17.



  PRACTICE PROBLEMS

                                 7. Name the following molecular compounds.
                                    a) S2Cl2
                                    b) CS2
For more practice with solving      c) SO3
problems, see Supplemental          d) P4O10
Practice Problems,
Appendix B.                      8. Write the formulas for the following
                                    molecular compounds.
                                    a) carbon tetrachloride
                                    b) iodine heptafluoride
                                    c) dinitrogen monoxide
                                    d) sulfur dioxide                       Vast deposits of
                                                                                     methane trapped
                                                                                     under high pressure in
                                                                                     the pores of ice have
Common Names                                                                         been located deep
                                                                                     under the ocean floor.
   A few inorganic molecular compounds have common names that all                    This ice is as cold as the
scientists use in place of formal names. Two of these compounds are                  ice in your freezer, but
water and ammonia. The chemical name for water is dihydrogen monox-                  it burns. If the methane
ide because each molecule contains two hydrogen atoms and one oxygen                 in the ice can be har-
                                                                                     vested, it may replace
atom. If you wanted to get a glass of water at a restaurant, would you ask
                                                                                     Earth’s dwindling sup-
for dihydrogen monoxide? Probably not, at least not if you were really               plies of fossil fuels
thirsty. Most people would not understand you because you used a name                because the deposits
that even chemists never use for water. Although the formal names of                 contain more than
both ionic and molecular compounds are simple to write once you learn                twice the amount of
the rules for the language of chemistry, there are good reasons for some-            energy as in all fossil
                                                                                     fuels combined.
times using common names. Which name you use will depend on your
audience.

                                                                        5.2   Molecular Substances           181
                                       The common acids are other examples of inorganic compounds that
                                    are known by common rather than formal names. Some names of com-
                                    mon acids and bases that you will use frequently in chemistry laboratory
                                    experiments are listed in Table 5.7. Although they often do not follow the
                                    rules you have been learning, they will soon become so familiar that their
                                    formulas and names will be easy to remember.


                                       Table 5.7 Names of Common Acids and Bases

                                       Formula                         Name
                                       Acids
                                          HCl           hydrochloric acid
                                          H2SO4         sulfuric acid
                                          H3PO4         phosphoric acid
                                          HNO3          nitric acid
                                          HC2H3O2       acetic acid (an organic compound)

                                       Bases
                                         NaOH           sodium hydroxide
                                         KOH            potassium hydroxide
                                         NH3            ammonia
Figure 5.18
Uses of Hydrocarbons
The structures of methane
(left) and propane (right) are      Naming Organic Compounds
shown here. Count the num-
ber of carbon atoms in each.          You have learned that most compounds that contain carbon are organic
Many hydrocarbons are used          compounds. Organic compounds make up the largest class of molecular
for fuel. Methane is the main       compounds known. This is because carbon is able to bond to other car-
component in natural                bon atoms in rings and chains of many sizes.
gas, and propane is
used in gas grills.




 Methane CH4                                                Propane C 3 H 8
       H                                                     H     H     H

  H     C     H                                         H    C     C     C    H
       H                                                     H     H     H


182           Chapter 5   Types of Compounds
   The name of even the most complex organic com-
pound is based on the name of a hydrocarbon, an                 Table 5.8 Hydrocarbons
organic compound that contains only the elements
hydrogen and carbon. Hydrocarbons occur naturally               Formula          Name
in fossil fuels such as natural gas and petroleum and              CH4          methane
are used mainly as fuels and the raw materials for                C2H6           ethane
making other organic compounds.                                   C3H8          propane
   A carbon atom can form four covalent bonds. In the             C4H10          butane
simplest hydrocarbon, methane, a single carbon is                 C5H12         pentane
bonded to four hydrogen atoms. Methane is the main                C6H14         hexane
component of the natural gas that you burn when you               C7H16         heptane
light a Bunsen burner. The next simplest hydrocarbon,             C8H18          octane
ethane, is formed when two carbon atoms bond to                   C9H20         nonane
each other as well as to three hydrogen atoms apiece.            C10H22         decane
The formulas and names of the first ten hydrocarbon
chains are shown in Table 5.8. Note that the names of
hydrocarbons are derived from the number of carbon
atoms in the molecules. Do you recognize any of these hydrocarbons? What
is propane used for? Figure 5.18 shows its structure and one common use.

Connecting Ideas
  Formulas represent the known composition of real substances, but just
because a formula can be written doesn’t mean the compound actually
exists. For example, you could easily write the formula HeP2, but no such
compound has ever been isolated. Compounds containing the noble gases
helium, neon, and argon have never been found. In the next chapter, you
will study the chemical changes that elements and compounds undergo
and learn how to represent these changes in the language of chemistry.



                                       SECTION REVIEW
Understanding Concepts                              3. Explain what allotropes are and give two
1. Write the formula for each of the following         examples.
   molecular compounds.                             Thinking Critically
   a) carbon monoxide                               4. Applying Concepts Explain, in terms of elec-
   b) phosphorus pentachloride                         tron structure, why carbon usually forms four
   c) sulfur hexafluoride                              bonds.
   d) dinitrogen pentoxide
   e) iodine trichloride                            Applying Chemistry
   f) heptane                                       5. Tank of Gas A tank of a substance delivered to
2. Write the name of the molecular compound            a factory is labeled C4H10. What is the name of
   represented by each formula.                        the substance in the tank? What is its most
                                                       likely use?
   a) BF3                d) IF7
   b) PBr5               e) NO
   c) C2H6               f) SiO2


            chemistryca.com/self_check_quiz                               5.2   Molecular Substances   183
                              CHAPTER 5 ASSESSMENT
                                          REVIEWING MAIN IDEAS
 5.1 Ionic Compounds                                           water solubility, and little or no ability to act
 ■    Binary ionic compounds are named by first                as electrolytes.
      naming the metal element and then the non-          ■    Seven elements occur naturally as diatomic
      metal element, with its ending changed to                molecules. They are hydrogen, nitrogen, oxy-
      -ide. Subscripts are used in formulas to indi-           gen, fluorine, chlorine, bromine, and iodine.
      cate how many atoms of each element are             ■    Some elements exist in different structural
      present in the compound.                                 forms called allotropes. Allotropes of an ele-
 ■    The position of an element in the periodic               ment have different properties.
      table indicates what charge its ions will have.     ■    Binary molecular compounds are named by
 ■    Polyatomic ions have fixed charges and can               writing the two elements in the order they are
      combine with ions of opposite charge to                  found in the formula, changing the ending of
      form ionic compounds. These compounds                    the second element to -ide, and adding Greek
      are named by writing the name of the                     prefixes to the element names to indicate how
      positive ion first and then the name of the              many atoms of each are present.
      negative ion.                                       ■    It is important to know both the formal and
 ■    Most transition elements can form two or                 common names of chemicals because both
      more positively charged ions. When naming a              are part of the language of chemistry.
      compound that contains a transition element,        ■    The naming system for organic compounds is
      the oxidation number of the transition ele-              based on the names of hydrocarbons.
      ment is indicated by a Roman numeral in
      parentheses.                                        Vocabulary
 ■    Hydrates are ionic compounds bonded to              For each of the following terms, write a sentence that shows
      water molecules. They are named by follow-          your understanding of its meaning.
      ing the name of the compound with a prefix          allotrope                      hygroscopic
      attached to the word hydrate to indicate how        anhydrous                      inorganic compound
      many water molecules are bound.                     binary compound                molecular element
                                                          deliquescent                   molecular substance
 5.2 Molecular Substances
                                                          distillation                   organic compound
 ■    Molecular substances have a greater variety of      formula unit                   oxidation number
      properties than do ionic compounds, but             hydrate                        polyatomic ion
      generally they have low melting points, low         hydrocarbon



                                                              2. Write the name for each of the following com-
      UNDERSTANDING CONCEPTS                                     pounds containing polyatomic ions.
 1. Which of the following substances are ionic                  a) Ca(C2H3O2)2         d) MgSO4
    and which are molecular?                                     b) NaOH                e) NaNO2
    a) magnesium sulfate d) ozone                                c) (NH4)2SO3 H2O f) Ca(OH)2
    b) hexane              e) cesium chloride                 3. Make a table comparing the properties of ionic
    c) carbon monoxide f) cobalt(II) chloride                    and molecular compounds.


184           Chapter 5   Types of Compounds            chemistryca.com/vocabulary_puzzlemaker
                          CHAPTER 5 ASSESSMENT
 4. The metals in the following compounds can             he or she add to a transparent glaze to achieve
    have various oxidation numbers. Predict the           this result?
    charge on each metal ion, and write the name
                                                       Chemistry and Technology
    for each compound.
                                                       17. Use the structural organization of graphite to
    a) FeCl3                d) SnBr4                       explain why it is a good lubricant.
    b) CuF2                 e) FeS
    c) AuBr3                f) Pb(C2H3O2)2             How It Works
 5. How can you tell if a compound is ionic or         18. How does concrete differ from cement?
    molecular by examining its formula?
 6. How is Na2HPO4 a substance with two differ-              THINKING CRITICALLY
    ent types of bonding?                              Designing an Experiment
 7. Write formulas for a bromine atom, ion, and        19. MiniLab 1 Design an experiment to deter-
    molecule.                                              mine the minimum amount of water required
 8. What happens to the composition of a hydrate           to change the color of the anhydrous cobalt
    when it is heated?                                     compound weather predictor.
                                                       Relating Cause and Effect
     APPLYING CONCEPTS                                 20. MiniLab 2 Why did vinegar soften the chicken
 9. In Samuel Coleridge’s poem The Rime of the             bone?
    Ancient Mariner, the mariner cried the follow-     Making Predictions
    ing while on his ship far from shore. “Water,
                                                       21. ChemLab Would you expect a warm or a cool
    water everywhere, and all the boards did
                                                           saturated solution of KNO3 in water to be a
    shrink/Water, water everywhere, nor any drop
                                                           better electrolyte? Explain.
    to drink.” What did he mean?
10. Predict the effect of increasing acidity of rain   Using a Table
    on the rate of formation of limestone caves.       22. The following table lists melting points for a
                                                           number of ionic compounds. Use a periodic
11. How could you determine quantitatively
                                                           table to help you answer the following questions.
    whether sodium hydroxide or calcium chloride
                                                           Do the melting points of the sodium and potas-
    is more deliquescent?
                                                           sium compounds increase or decrease as you
12. Why don’t the noble gases form compounds               move down Group 17? What does this suggest
    easily?                                                about the strength of the ionic bonds between
13. How could you determine quantitatively                 these metals and the Group 17 nonmetals?
    whether the ionic compound table salt or the
    molecular compound table sugar is more solu-              Compound                  Melting Point (°C)
    ble in water?                                                 NaCl                          804
14. Explain why most elements do not occur natu-                  NaI                           651
    rally in their pure state.
                                                                  KCl                           773
Everyday Chemistry                                                KBr                           730
15. What is hard water and how is it treated?                     NaF                           993
Art Connection                                                     KI                           680
16. Suppose an artisan wanted to coat a clay vessel               NaBr                          755
     with a faint pink glaze. What material should

             chemistryca.com/chapter_test                                   Chapter 5   Assessment           185
                               CHAPTER 5 ASSESSMENT
Applying Concepts
23. Mercury(I) is unusual in that it often forms an
                                                                    WRITING IN CHEMISTRY
    ion that links with another mercury(I) ion.               28. Write a set of descriptions comparing the
    Thus, two mercury(I) ions are linked together in              structures of a soccer ball, a geodesic dome,
    a single unit. What is the charge on this double              and buckminsterfullerene. Is their similarity a
    ion? Write the formula for the compound that is               coincidence?
    formed from this double ion and chlorine.
                                                                    PROBLEM SOLVING
        CUMULATIVE REVIEW                                     29. Different ions and elements that have the same
24. How are physical changes different from chem-                 electronic structure are said to be isoelectronic.
    ical changes? (Chapter 1)                                     Na is isoelectronic with Ne; both have ten
25. How does the atomic number compare with                       electrons, including eight valence electrons.
    the number of electrons in a neutral atom?                    Mg2 also is isoelectronic with Na and Ne.
    (Chapter 2)                                                   Write the symbols for two ions or elements
26. How can the periodic table be used to deter-              30. a) Write the formulas for phosphorus trioxide
    mine the number of valence electrons in an                       and phosphorus pentoxide.
    element? (Chapter 3)                                          b) What percent of the atoms in phosphorus tri-
                                                                     oxide are phosphorus? What percent are oxygen?
                                                                  c) What percent of the atoms in phosphorus
        SKILL REVIEW                                                 pentoxide are phosphorus? What percent are
27. Making and Using Graphs Using the follow-                        oxygen?
    ing data, construct a graph of melting point              31. Vitamin C is a covalent compound with the
    versus number of carbons and a graph of                       formula C6H8O6. If a one ounce serving of
    water solubility versus number of carbons.                    potato chips provides you with 30 percent of
    What is the relationship between chain length                 the recommended daily value of Vitamin C,
    (number of carbon atoms) and melting point?                   how many servings of the potato chips will you
    Can you explain why? How are chain length                     have to eat to get 100 percent of the recom-
    and water solubility related?                                 mended daily value of this vitamin?
                                                              32. Rubbing alcohol contains the covalent com-
   Number of           Melting Point       Water Solubility       pound isopropanol. The most common form
  Carbon Atoms              (°C)           (g per 100 mL)         of rubbing alcohol available in drugstores con-
      1 (methane)             183                0.0024           tains 70 percent isopropanol and 30 percent
      2 (ethane)              172                0.0059           water by volume.
                                                                  a) What is the ratio of isopropanol to water in
      3 (propane)             188                0.012
                                                                     the common form of rubbing alcohol?
      4 (butane)              138                0.037            b) What is the volume of isopropanol in
      5 (pentane)             130                0.036               200 mL of rubbing alcohol? What is the
      6 (hexane)               95                0.0138
                                                                     volume of water?
                                                              33. Impurities weaken the interparticle forces in a
      7 (heptane)              91                0.0052
                                                                  molecular substance and create irregularities
      8 (octane)               57                0.0015           in the crystal structure. What effect do impu-
      9 (nonane)               54               insoluble         rities have on the melting point of a molecular
                                                                  substance?
  10 (decane)                  30               insoluble


186            Chapter 5   Types of Compounds
                          Standardized Test Practice
1. A binary compound contains                            Formula and Names of Common Compounds
   a) two elements.                                                 Containing Nitrogen
   b) two ions.                                          Formula Molecular Compound                Common
   c) two oxidized elements.                                            Name                        Name
   d) two bonds.
                                                            ?       Nitrogen monoxide            Nitrogen
2. An oxidation number is                                                                        monoxide
   a) the number of electrons an atom will lose.          NH3                  ?                 Ammonia
   b) the number of electrons an atom will gain.            ?       Dinitrogen tetrahydride Hydrazine
   c) the overall charge of an atom.                      N2O                  ?                 Nitrous oxide
   d) the overall charge of an ion.                                                              (Laughing gas)
3. Which of the following choices is an example           NO2                  ?                 Nitrogen
   of a polyatomic ion?                                                                          dioxide
   a) CO2                 c) MnO4
   b) Mg 2                d) NaCl                       Using the table above to answer questions 8–9.
                                                         8. What is the molecular compound name for
4. What is the correct chemical formula for the             laughing gas?
   ionic compound formed by the calcium ion
   (Ca 2+) and the acetate ion (C2H3O2 )?                   a) mononitrogen dioxide
                                                            b) nitrogen dioxide
   a) CaC2H3O2                                              c) dinitrogen monoxide
   b) CaC4H6O8                                              d) dinitrogen oxide
   c) (Ca)2C2H3O2
   d) Ca(C2H3O2)2                                        9. What is the molecular formula of hydrazine?
5. Yttrium, a metallic element with atomic number           a) N4H2
   39, will form                                            b) N2H4
                                                            c) N2(OH)4
   a) positive ions.                                        d) N4(OH)2
   b) negative ions.
   c) both positive and negative ions.                  10. Hydrocarbons are most commonly used as
   d) no ions at all.                                       a) acids.
6. Copper (II) sulfate has the chemical formula             b) bases.
                                                            c) fuels.
   a) CuSO4                                                 d) explosives.
   b) Cu2SO4
   c) Cu2(SO4)2
   d) CuS2O8
                                                                            Test Taking Tip
7. Which statement is NOT true about
   allotropes?                                             When Eliminating, Cross It Out              Consider
   a) Allotropes contain only one element.                 each answer choice individually and cross out
                                                           choices you’ve eliminated. If you can’t write in the
   b) Allotropes have different oxidation numbers.         test booklet, use the scratch paper. List the answer
   c) The properties of allotropes are different.          choice letters on the scratch paper and cross them
   d) Allotropes have different molecular structures.      out there. You’ll save time and stop yourself from
                                                           choosing an answer you’ve mentally eliminated.



            chemistryca.com/standardized_test                                Standardized Test Practice           187

				
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