THE PERIODIC TABLE AND THE ELEMENTS

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					                          CHEMISTRY 30S – MODULE 3
                       CHEMICAL REACTIONS

LESSON 2  Writing Formulas and Naming Compounds

When you have completed this lesson, you will be able to:

      - Write formulas and names for chemical compounds using IUPAC rules.


Chemical Formulas

A chemical formula is a shorthand method to represent compounds that uses the
elements' symbols and subscripts. The chemical formula gives the following information:
      The different elements in the compound.
      The number of atoms of each element in the compound.
For example,




Figure 1 shows that water contains hydrogen, H, and oxygen, O, atoms. The subscripts
that follow each element tell how many atoms of that element are in the atoms of that
compound. The subscript following the symbol for hydrogen is 2, telling us there are two
hydrogen atoms in each water molecule. There is no subscript after the symbol for
oxygen; this shows that there is only one atom of oxygen in a water molecule. Chemists
do not write the number one as a subscript when only one atom of that kind of element
exists in the compound.
Figure 2 shows a more complex chemical formula. The compound contains calcium, Ca,
phosphorous, P, and oxygen, O. The subscript after the symbol for calcium indicates
there are 3 atoms of calcium in one molecule. The subscript, 2, outside the bracket
indicates that the whole group of atoms inside the bracket, called a POLYATOMIC
ION, exists twice in every atom. Therefore, every subscript inside the bracket is to be
multiplied by 2. This means there are 2 x 1 = 2 atoms of phosphorous and 2 x 4 = 8 atoms
of oxygen in each molecule of Ca3(PO4)2.
TALC                                                                                2010


Ionic Compounds

An ion is a charged particle. An ion is formed when a neutral atom gains or loses
electrons. When an atom loses electrons the number of positively charged protons
exceeds the number of negatively charged electrons, resulting in a positively charged
particle. Positively charged ions are called cations. A negative ion, called an anion, is
formed when an atom gains one or more electrons. Since electrons are negatively
charged, the resulting ion has an overall negative charge.
Ionic compounds are formed when two or more oppositely charged ions are attracted to
each other. This chemical attraction is called a chemical bond. An ionic bond is formed
when a negatively charged ion is attracted to a positively charged ion. Ions combine
together so that their charges add up to zero -- neutral.
Ionic compounds are usually made when metal ions (recall they are from the left side of
the zigzag line) and non-metal ions (nonmetals are on the right side of the periodic table)
combine. For example,
NaCl – sodium chloride
KBr – potassium bromide
CaO – calcium oxide
Fe2O3 – iron oxide

These ones are a little different:

CuSO4 – copper sulfate
Ca3(PO4)2 – calcium phosphate

Not all the ions are found in the ion chart. For the ions not found in the chart, we can use
the periodic table to predict their charge. The alkali metals, group 1, will each tend to lose
1 electron to produce ions with a 1+ charge. The alkaline earth metals, group 2, tend to
form ions with a 2+ charge. Many of the transition metals, because of their electron
arrangement, tend to form more than one ion charge. The chalcogens, group 6, tend to
form 2– ions and the halogens, group 7, tend to form 1– ions.


  Naming Binary Ionic Compounds

  A binary compound contains two different kinds of elements. There can be more
  than one atom of each element in a binary compound. Binary ionic compounds
  usually contain one kind of metal ion combined with one kind of non-metal ion.
  Metal ions are usually positively charged and non-metal ions are usually negatively
  charged.
  When naming an ionic compound from its formula follow the rules below:
              1. When naming, the cation (positive ion) is named first, followed by the


CHEM 30S                      Module 3 Chemical Reactions Lesson 2                      2
TALC                                                                             2010


                 anion (negative ion).
             2. Write the full name of the metallic element (positive ion).
             3. Write the name of the non-metallic element (negative ion) and change
                the ending to "-ide".
 Example 1. Write the name of NaCl.

 Solution.
 Step 1: Name the first element.
 Na = sodium
 Step 2: Name the second element and change the ending to "-ide".
 Cl = chlorine = chloride
 The name of the compound is sodium chloride.
 Example 2. Write the name of Mg3P2.

 Solution.
 Step 1: Name the first element.
 Mg = magnesium
 Step 2: Name the root of the second element and add "-ide".
 P = phosphorous = phosphide
 The name of the compound is magnesium phosphide.
 Practice:
 Write the names for each of these formulas:
 1. CaI2
 2. Na4C
 3. SrBr2
 4. Li3N
 5. BaS


 Writing Binary Ionic Formulas, Given the Compound's Name

 You will need the ion chart given to you in class to write the formulas of the ionic
 compounds. Remember the formula contains the symbols of the elements and the
 subscripts indicating the number of atoms of each element.
 There are two methods for determining the formula of a compound, but the following
 points must hold true:
    1. The formula must have cation first (positively charged ion, usually the metal)



CHEM 30S                    Module 3 Chemical Reactions Lesson 2                        3
TALC                                                                             2010


        followed by the anion (negatively charged ion, the non-metal).
    2. The sum of the charges of the ions must be zero. That is, the number of
       positive charges must equal the number of negative charges.
    3. You may not change the charge of the ions to make the ion charges equal zero.

 Method 1 – Lowest Common Multiple
             1. Write the symbols for the ions involved.
             2. Determine the lowest whole number ratio that will give an overall net
                charge of zero. That is, the number of positive charges must equal the
                number of negative charges.
 Example 3. Write the formula for aluminum oxide.
 Solution.
 Step 1: Write the ions and their charges from the ion chart.
 Al3+ and O2–
 Step 2: Determine the lowest common multiple.
 In this case, the lowest common multiple for 3 and 2 is 6. To get 6 positive charges,
 we would need 2 aluminums (2 x 3+ = 6+). To get 6 negative charges we need 3
 oxygens (3 x 2– = 6–). We make these numbers the subscripts for each.
 The formula for aluminum oxide is Al2O3.
 Method 2 – The "Criss-Cross" Method
 This method accomplishes the same as the lowest common multiple method-the total
 charge of the compound is zero. Some students find this method simpler.
             1. Write the ions and their charges side by side.
             2. Make the number of the charge of one ion the subscript of the other ion
                (omitting the + or – sign). Remember we do not write the number one
                as a subscript.
             3. Reduce all subscripts to their simplest form, if necessary.
 Example 4. Write the formula for aluminum oxide.
 Solution.
 Step 1: Write the ions and their charges from the ion chart.
 Al3+ and O2–
 Step 2: Make the number of the charge of one ion the subscript of the other ion.




CHEM 30S                    Module 3 Chemical Reactions Lesson 2                     4
TALC                                                                           2010




 Example 5. Write the formula for barium fluoride.

 Solution.
 Step 1: Write the ions and their charges from the ion chart.
 Ba2+ and F–
 Step 2: Make the number of the charge of one ion the subscript of the other ion.




 Note: The charge on the fluoride ion is 1–. Since IUPAC rules do not write the number
 one as a subscript, we leave the barium without a subscript.

 Practice:
 Write the formulas for each of these:
 1. strontium chloride
 2. aluminum iodide
 3. calcium sulfide
 4. aluminum nitride
 5. sodium oxide

  Writing Formulas for Binary Ionic Compounds Having Metal Ions
  With Multiple Charges

 Most of the transition metals have more than one possible ion charge. For example:
                              Ion          Possible Ion Charges
                             Copper                1+, 2+
                              Iron                 2+, 3+
                             Cobalt                2+, 3+
                           Chromium                2+, 3+
                              Lead                 2+, 4+
                               Tin                 2+, 4+

 The reason for this is beyond the scope of this course. You can do an internet search
 if you are dying of curiosity about this 



CHEM 30S                    Module 3 Chemical Reactions Lesson 2                   5
TALC                                                                           2010


 In 1919, Alfred Stock (1876 – 1946), a German chemist, suggested using numbers to
 indicate the charge of the ions. Prior to this the ions were given different names
 based upon their charge. For example, the Cu+ ion was called cuprous and the Cu2+
 ion was called cupric. However, the Fe2+ ion was ferrous and the Fe3+ ion was ferric.
 Since the charges were not always the same, the "–ic" and "–ous" suffixes caused
 some confusion, especially for chemistry students. Today, the Stock naming system
 uses Roman numerals following the metal ion's name to indicate the ion's charge.
 For example,
 Copper (I) = Cu+
 Copper (II) = Cu2+
 Iron (II) = Fe2+
 Iron (III) = Fe3+
 As a general rule, all metals have more than one ion charge except group one and
 two metals, silver, cadmium, zinc, and aluminum. Unless the metal is one of these
 use the Roman numeral.
 Example 6. Write the formula for iron (III) chloride

 Solution.
 Step 1: Write out the ions.
 iron (III) and chloride
 Fe3+ and Cl–
 Step 2: Balance or “criss-cross” the charges.
 FeCl3
 Example 7. Write the formula for lead (IV) sulfide.

 Solution.
 Step 1: Write the ions.
 lead (IV) and sulphide
 Pb4+ and S2–
 Step 2: Balance or “criss-cross” the charges.
 Pb2S4
 Step 3: Reduce the subscripts.
 Both subscripts are divisible by 2.
 PbS2

  Naming Binary Ionic Compounds Having Metal Ions With
  Multiple Charges – Stock System

 We name in a very similar manner as those ions with a single ion charge, except we


CHEM 30S                   Module 3 Chemical Reactions Lesson 2                      6
TALC                                                                             2010


 must figure out the charge on the metal ion since it has more than one possibility,
 and then we must write the roman numeral in brackets after the cation in the name.
 To determine the charge on the metal ion, I will give detailed instructions as
 explained on WebCT – however I prefer to say “you figure out the charge on the
 cation by looking at what you know about the anion and what the charge is on it,
 how many there are, etc” – the compound must be neutral. Most of you can probably
 do it without reading the following section – try examples 8 and 9 first if you feel
 confident, and if you get the answer right without following the example, then skip
 the reading along with it.
 The steps are:
 1. Write the name of the ions.
 2. Multiply the charge of the anion by its subscript.
 3. Divide this number by the subscript of the metal ion. The result is the charge on
 the metal ion.
 Example 8. Write the name for CoBr2.

 Solution.
 Step 1: Write the names of the ions. (including the charge of the anion)
 cobalt bromide
 Co? Br 1–
 Step 2: Multiply the charge of the bromide by its subscript then divide by the
 subscript for cobalt.
 The charge on the bromide is 1– so, 1 x 2 = 2 (we will ignore the charge)
 The subscript for cobalt is one (CoBr2). This means the charge on the cobalt is 2+
 (since the total positive charge and total negative charge must add to zero).
 Step 3: Write the name, indicating the charge of cobalt (2+) using roman
 numerals.
 cobalt (II) bromide
 Example 9. Write the name for Fe2O3.

 Solution.
 Step 1: Write the names of the ions.
 iron oxide
 Fe? O2–
 Step 2: Multiply the charge of the oxide by its subscript then divide by the
 subscript for cobalt.
 The charge on the oxide is 2– so, 3 x 2 = 6 (we will ignore the charge)
 The subscript for iron is 2. This means the charge on the iron is 6 ÷ 2 = 3+.
 Step 3: Write the name(writing the charge of iron (3+) in roman numerals)



CHEM 30S                   Module 3 Chemical Reactions Lesson 2                       7
TALC                                                                            2010


  Fe3+ O2–
  iron (III) oxide

Writing Names for Compounds Containing Polyatomic Ions

Some ions are composed of several atoms joined covalently. These are called
polyatomic ions (poly = many). An example of a polyatomic ion is PO43–, the phosphate
ion

Although the polyatomic ions have more than one atom, we will name these like binary
compounds. The charge for polyatomic ions is for the whole group of atoms not just for
the atom written last. DO NOT change the subscripts of polyatomic ions; if you change
the subscripts you change the identity of these ions.
When indicating the presence of more than one polyatomic ion in a compound, we use
parenthesis around the polyatomic ion, followed by its subscript. For example, the
compound Al(C2H3O2)3 has an aluminum ion and 3 acetate ions. Placing the acetate ion
in parenthesis and following it with the subscript 3 indicates there are 3 acetate ions.
Example 10. Write the name for KNO3.

Solution.
Step 1: Identify the cation.
K+, from group 1, does not have a variable charge, so we don't use a Roman numeral. The
name of the ion is potassium.
Step 2: Identify the anion.
NO3– is the nitrate ion.
Step 3: Write the name of the cation first, followed by the anion.
The name of the compound is potassium nitrate.
Example 11. Write the name of Cu3(PO4)2.

Solution.
Step 1: Identify the cation.
Copper, but Copper is one of the ions with a variable charge so we must use a Roman
numeral to indicate the charge on the ion. To determine the charge of the copper ion, we
must look at the anion.
Step 2: Identify the anion.
The anion is the phosphate ion (PO43–). The parenthesis followed by the number 2
indicates there are 2 phosphate ions in this compound. From our chart we know each
phosphate has a charge of 3– for a total charge of 2 x 3 = 6.
Step 3: Determine the charge of the cation.



CHEM 30S                       Module 3 Chemical Reactions Lesson 2                 8
TALC                                                                               2010


The total charge of the anions is 6, so the charge of the copper is 6 ÷ 3 (subscript on the
Cu) = 2.
Step 4: Write the name of the cation first, followed by the anion.
The name of the compound is copper (II) phosphate.
Example 12. Write the name of Hg2Cl2.

Solution.
Step 1: Identify the cation.
The cation is mercury. Mercury is a special case for a metal ion. The mercury (I) ion is
like a polyatomic ion. The two mercury atoms act together for a total 2+ charge. The
mercury (I) ion is Hg22+.
Step 2: Identify the anion.
The anion is the chloride ion.
Step 3: Write the name of the cation first, followed by the anion.
The name of the compound is mercury(I) chloride.
Example 13. Write the name of NH4SCN.

Solution.
Step 1: Identify the cation.
The cation is NH4+, the ammonium ion.

Step 2: Identify the anion.
The anion is SCN–, the thiocyanate ion.
Step 3: Write the name of the cation first, followed by the anion.
The name of the compound is ammonium thiocyanate.
Practice:
Write the names for each of these formulas:
1. Ni(NO3)2
2. MgSO4
3. KMnO4
4. Fe(OH)3
5. Ca(C2H3O2)2
6. Ag2CrO4
7. PbCO3
8. Sn(SO4)2
9. Na2HPO4
10. NH4NO2




CHEM 30S                       Module 3 Chemical Reactions Lesson 2                    9
TALC                                                                             2010


Naming Binary Covalent Compounds

Non-metals tend to combine chemically by sharing electron pairs. These bonds are
known as covalent bonds. Neutral compounds made of atoms joined covalently are
called molecular or covalent compounds.
We name covalent compounds differently than ionic compounds. We must indicate the
number of each element by adding a prefix in front of the element's name.
The prefixes are:
one = mono
two = di
three = tri
four = tetra
five = penta
six = hexa
seven = hepta
eight = octa
nine = nona
ten = deca
Naming covalent compounds:
Step 1: Name the first element in full using a prefix only when there are two or more
        of that element. That is, omit "mono" if only one of that element is in the
        compound.
            e.g. NO is nitrogen monoxide, but N2O is dinitrogen monoxide

Step 2: Name the second element and end in "-ide". Use prefixes to indicate the
        number of that element (including mono).


Step 3: Write the name of the compound writing the substance found more to the left
        on the periodic table first.
:There are two exceptions to the naming rules. Here the common names for the
compounds are used:
H2O = water
NH3 = ammonia
Example 18. Write the name for CO2.

Solution.
This is a covalent compound since it is made of two non-metal atoms.

Step 1: Name the first atom with prefixes.
There is only one carbon atom. We omit the "mono-" for the first element, so the first part


CHEM 30S                     Module 3 Chemical Reactions Lesson 2                   10
TALC                                                                           2010


of the name is carbon.
Step 2: Name the second element using prefixes and end in "-ide".
The second element is oxygen. There are two oxygen atoms, so the second part of the
name is dioxide.
Step 3: Write the name of the compound writing the substance found more to the
left on the periodic table first.
The name of CO2 is carbon dioxide.
Example 19. Write the name for N2O4.

Solution.
Step 1: Name the first atom with prefixes.
There are two nitrogens, so the first part of the name is dinitrogen.
Step 2: Name the second element using prefixes and end in "-ide".
The second element is oxygen. There are four oxygen atoms, so the second part of the
name is tetraoxide.
Step 3: Write the name of the compound.
The name of N2O4 is dinitrogen tetraoxide.
Practice:
Write the names for each of these formulas:
1. NI3
2. CO
3. SF6
4. P4O10
5. N2O5

Writing Formulas for Binary Covalent Compounds

Writing formulas for binary covalent compounds involves the following steps:
Step 1: Write the symbol for the first element followed by the subscript
         indicated by the prefix.
Step 2: Write the symbol of the second element followed by the prefix
        indicated by its prefix.
Do not reduce!
Example 20. Write the formula for dinitrogen monoxide.

Solution.
Step 1: The first symbol is N and its subscript is 2.
Step 2: The second symbol is O and its subscript is 1.



CHEM 30S                     Module 3 Chemical Reactions Lesson 2                11
TALC                                                                               2010


The formula for dinitrogen monoxide is N2O.
Example 21. Write the formula for sulphur hexafluoride.

Solution.
Step 1: The first symbol is S and its subscript is 1.
Step 2: The second symbol is F and its subscript is 6.
The formula for sulphur hexafluoride is SF6.
Practice:
Write the names for each of these formulas:
1. dichlorine heptaoxide
2. carbon disulphide
3. silicon tetrafluoride
4. diphosphorous pentaoxide
5. tetraarsenic decaoxide


Diatomic Molecules

Remember that some elements do not exist as single atoms in the natural world. These
elements exist as pairs of atoms joined covalently, called diatomic molecules. The
elements that exist as diatomic molecules are hydrogen gas (H2), oxygen gas (O2),
fluorine gas (F2), chlorine gas (Cl2), bromine gas or liquid (Br2) and solid iodine (I2) and
astatine (As2). You need to remember these – remember Robyn’s “Seven Rule” – if you
missed class you need to figure out why I call it that; there are 3 reasons. Hand in your
reasons next class 


ANSWER KEY

Answers to Practice Questions Page 3
1. calcium iodide
2. sodium carbide
3. strontium bromide
4. lithium nitride
5. barium sulfide

Answers to Practice Questions Page 5
1. SrCl2
2. AlI3
3. CaS
4. AlN
5. Na2O

Answers to Practice Questions Page 9


CHEM 30S                     Module 3 Chemical Reactions Lesson 2                     12
TALC                                                                     2010


1. nickel (II) nitrate
2. magnesium sulfate
3. potassium permanganate
4. iron (III) hydroxide
5. calcium acetate
6. silver chromate
7. lead (II) carbonate
8. tin (IV) sulfate
9. sodium dihydrogen phosphate
10. ammonium nitrite (not nitrate)

Answers to Practice Questions Page 11
1. nitrogen triiodide
2. carbon monoxide
3. sulphur hexafluoride (or sulfur)
4. tetraphosphorous decaoxide
5. dinitrogen pentaoxide

Answers to Practice Questions Page 12
1. Cl2O7
2. CS2
3. SiF4
4. P2O5
5. As4O10


Source:
Manitoba WebCT for much of the content and all the practice questions.




CHEM 30S                    Module 3 Chemical Reactions Lesson 2           13

				
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