Sections Covalent Crops
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.
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
• plastic freezer bag containing the following
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.
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
5.1 Ionic Compounds
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
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
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
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
ments. Binary ionic compounds can contain more than one ion of each
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-
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
The noble gases each have eight electrons in their outer-energy levels.
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.
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.
156 Chapter 5 Types of Compounds
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
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–
Table 5.1 Ionic Charges of Representative Elements
Group Number Oxidation Number Examples
11 1 Li , Na , K
12 2 Mg2 , Ca2
13 3 B3 , Al3
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
Analyze • in Groupis16 and has1, sooxidation number ofnumber of 1 . Sulfur is
Sodium in Group
it has an oxidation
Set Up • Write the symbols for sodium and sulfur ions in formula form, placing
the positive ion first.
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.
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
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-
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, 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
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
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
Analyze • Lithiumcarbonate ion1,has its2ioncharge,1andcharge. According to Table
is in Group so
its structure is CO .3
Set Up • Write the symbols for lithium carbonate in formula form.
Solve • Determine charges. In this case,lithium ions thecarbonateand negative
the correct ratio of
the sum of
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
Analyze • Aluminumto Table 5.2, 13 and has ion oxidation charge. of 3 .
is in Group
has a 2
Set Up • Write the symbols for aluminum sulfate in formula form.
Al SO 4
Solve • Determine charges. In this case,aluminumofions to sulfateand negative
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
3. Write the formula for the compound made from each of the follow-
a) ammonium and sulfite ions
For more practice with solving b) calcium and monohydrogen phosphate ions
problems, see Supplemental
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
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
162 Chapter 5 Types of Compounds
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
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
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)
Cr 6 chromium(VI)
Cobalt Co2 cobalt(II) CrO
Copper Cu copper(I)
Gold Au gold(I)
Iron Fe2 iron(II)
Manganese Mn2 manganese(II)
Mercury Hg mercury(I)
Nickel Ni2 nickel(II)
Ni3 nickel(III) CrO3
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
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
c) gold(III) and cyanide
Appendix B. d) lead(II) and sulfide
6. Write the names of the following compounds.
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
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.
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.
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
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
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
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.
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.
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
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
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
5.2 Molecular Substances
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
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
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
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
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
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
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
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
spatula A B C D
4 small beakers (50- or 100-mL)
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?
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
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
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.
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.
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.
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-
Cl Cl ˇ Cl Cl allos (GK) other
Hydrogen, fluorine, bromine, and iodine molecules also are formed by the tropos (GK) way,
sharing of a single pair of electrons. Two oxygen atoms share two pairs of
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
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
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
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
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.
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.
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—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
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
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
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.
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
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.
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.
7. Name the following molecular compounds.
For more practice with solving c) SO3
problems, see Supplemental d) P4O10
Appendix B. 8. Write the formulas for the following
a) carbon tetrachloride
b) iodine heptafluoride
c) dinitrogen monoxide
d) sulfur dioxide Vast deposits of
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
times using common names. Which name you use will depend on your
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
HCl hydrochloric acid
H2SO4 sulfuric acid
H3PO4 phosphoric acid
HNO3 nitric acid
HC2H3O2 acetic acid (an organic compound)
NaOH sodium hydroxide
KOH potassium hydroxide
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.
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.
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
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
■ 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.
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
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.
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
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
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
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.
a) CuSO4 d) explosives.
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