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.
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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
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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)
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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.
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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 
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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
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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)
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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.
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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
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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
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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.
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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
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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.
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