5.7
Names and Formulas of Ionic Compounds
Blue vitriol, cinnabar, Glauber’s salt (Figure 1). These unusual names may
sound like the ingredients in a magical potion. However, they are actually
the traditional names of three chemicals commonly found in chemistry
labs. These names were developed centuries ago, when few chemicals
were known. Today, the number of known chemicals has grown to over
10 million! To keep track of them all, chemists have developed a systematic
method of naming chemicals. The International Union of Pure and Applied
Chemistry (IUPAC) is the organization that decides how chemicals will be
named. A common naming system allows scientists around the world to
communicate with each other without misunderstandings.
(a)
Naming Ionic Compounds
Many ionic compounds are made up of two elements: a metal and a
non-metal. It is therefore logical that the names of ionic compounds have two
parts. The first part refers to the metal ion in the compound and the second
part to the non-metal ion. Remember that the name of the metal ion remains
the same as the name of the neutral metal atom (Table 1) but that the ending
of the name of the second ion—the non-metal—changes to “ide.”
(b)
Table 1 Examples of Naming Ionic Compounds
Metal
Metal ion
Non-metal
Non-metal ion
Compound
magnesium
magnesium ion
chlorine
chloride ion
magnesium chloride
aluminum
aluminum ion
oxygen
oxide ion
aluminum oxide
Table 2 gives the ion names of common non-metals found in ionic
compounds. Remember, from Section 5.5, that all non-metals form negatively
charged ions: anions.
Table 2 Names and Charges of Common Anions
(c)
Figure 1 Some chemicals have been
known for centuries, such as (a) blue
vitriol, (b) cinnabar, and (c) Glauber’s
salt. At the time of their discovery, there
was no organized way of naming them.
196
Name of element
Name of ion
Ionic charge
Ion symbol
fluorine
fluoride ion
−1
F−
chlorine
chloride ion
−1
Cl −
bromine
bromide ion
−1
Br −
iodine
iodide ion
−1
I−
oxygen
oxide ion
−2
O2 −
sulfur
sulfide ion
−2
S2 −
nitrogen
nitride ion
−3
N3 −
phosphorus
phosphide ion
−3
P3 −
Chapter 5 • Chemicals and Their Properties
NEL
Writing Chemical Formulas
of Ionic Compounds
When elements form ionic compounds, electrons move from metal atoms
to non-metal atoms. The resulting charged ions attract other ions of the
opposite charge until the charges balance out. The compound that forms is
electrically neutral. In other words, in the compound, the total number of
positive charges must equal the total number of negative charges. This basic
idea helps us determine the chemical formulas of ionic compounds.
To determine the chemical formula of an ionic compound, you must first
figure out the correct number of ions required to produce an electrically
neutral compound. The compound’s total ion charge (the negative and
positive ion charges added together) must equal zero. Here is a strategy
that will help.
SAMPLE PROBLEM 1
Chemical Formula of an Ionic Compound
What is the chemical formula of magnesium chloride?
Step 1 Write the symbols of the elements, with the metal on the left-hand side and
the non-metal on the right-hand side.
LEARNING TIP
The Zero-Sum Rule
The sum of all charges in the chemical
formula of the compound must
equal zero. This model may help you
understand this idea. It shows that two
chloride ions (triangles) are needed to
“complete” the rectangle. The rectangle
represents the smallest number of ions
that must combine to give an overall
charge of zero. The ratio of ions in this
rectangle is the same as the ratio of
ions in the chemical formula of the
compound.
MgCl
MgCl
22
Mg
Cl
Step 2 Add the ionic charge of each ion above the symbol.
+2
−1
Mg
Cl
––
ClCl
Step 3 Determine how many ions of each type are required to bring the total charge
to zero. The sum of all charges in the compound must equal zero.
Total ionic charge:
1(+2)
+
2+2+
Mg
Mg
––
ClCl
2+2+
Mg
Mg
––
ClCl
2(−1) = 0
––
ClCl
Mg
Cl
Step 4 Write the chemical formula using the (red) coefficients in front of each bracket
as subscripts.
Mg1Cl2
Step 5 Do not write the subscript “1” in chemical formulas because the symbol itself
represents one ion.
The chemical formula for magnesium chloride is MgCl2
SAMPLE PROBLEM 2
Chemical Formula of an Ionic Compound
What is the chemical formula of aluminum oxide (Figure 2)?
Step 1 Write the symbols of the metal and non-metal elements.
Al
O
Step 2 Add the ionic charge of each ion above the symbol.
+3
−2
Al
O
Step 3 Determine the number of ions required to bring the total charge to zero.
2(+3)
+
3(−2) = 0
Al
O
The chemical formula of aluminum oxide is Al2O3.
NEL
Figure 2 A layer of aluminum oxide forms a
protective coating over the metal underneath.
5.7 Names and Formulas of Ionic Compounds
197
The Crisscross Method for Determining Formulas
WRITING TIP
Writing a Summary
Sometimes a text describes a process
or procedure, such as the Crisscross
Method. In your summary, organize
ideas and information in the same order
as in the original text.
Now that you understand the meaning of the symbols and numbers in
chemical formulas, try using a shortcut known as the “crisscross” method.
SAMPLE PROBLEM 3
Using the Crisscross Method
What is the chemical formula of magnesium chloride?
Step 1 Write the symbols of the elements and their ionic charges.
Mg2+ Cl−
Step 2 Crisscross the numbers of the ionic charges so that they now become subscripts.
Cl−
Mg2+
MgCl2
The chemical formula of magnesium chloride is MgCl2.
Practice
What is the chemical formula of aluminum sulfide?
Occasionally, this method of finding the chemical formula of a compound
gives you an answer with two identical subscripts. For example, the chemical
formula of aluminum nitride appears to be Al3N3. You know that the
chemical formula of an ionic compound is always the simplest possible ratio
of the ions, so Al3N3 must be simplified to AlN.
Elements with Multiple Ionic Charges
Figure 3 Because iron has two
different ionic charges, it makes two
different compounds with chlorine:
iron(II) chloride (left) and iron(III)
chloride (right).
198
Chemists have discovered, through experiments, that there are two stable
cations of iron: Fe2+ and Fe3+. These ions are named like other cations, except
that Roman numerals in rounded brackets are used to indicate the ionic charge
(not the number of ions) of the metal. The Fe2+ ion is called “iron(II) ion,” and
the Fe3+ ion is called “iron(III) ion.” Because of this, iron may form two different
compounds with a particular anion. For example, there are two different
compounds of iron and chlorine: FeCl2 and FeCl3. Each compound has its own
distinct properties: solid FeCl2 is light green, while FeCl3 is yellowish-brown
(Figure 3). Several other metals, besides iron, form multiple cations. In each
case, two or more compounds can be formed with the same anion (Table 3).
Table 3 Names and Multiple Ionic Charges for Common Metals
Metal
Chemical symbol
of element
Chemical symbols
of ions
Names of ions
copper
Cu
Cu+
Cu2+
copper(I)
copper(II)
iron
Fe
Fe2+
Fe3+
iron(II)
iron(III)
lead
Pb
Pb2+
Pb4+
lead(II)
lead(IV)
manganese
Mn
Mn2+
Mn4+
manganese(II)
manganese(IV)
tin
Sn
Sn2+
Sn4+
tin(II)
tin(IV)
Chapter 5 • Chemicals and Their Properties
NEL
Naming Compounds Involving Elements with Multiple
Ionic Charges
Naming compounds containing elements with multiple ionic charges involves
the same procedure as before. You must also determine the ionic charge of
the metal so that you can include it in the name. For example, FeCl2 is called
iron(II) chloride because the ionic charge of iron is +2. Similarly, FeCl3 is
called iron(III) chloride because the ionic charge of iron is +3.
SAMPLE PROBLEM 4
Considering Multiple Ionic Charges
LEARNING TIP
Specifying the Charge
The name of CuCl2 is written “copper(II)
chloride.” This is read as “copper two
chloride.” Remember that the “two”
refers to the charge on the copper ion,
not to the number of ions of copper in
the compound.
Write the chemical name of CuBr2. Note that copper could have either of two ionic
charges (Table 3).
Step 1 As you do not know whether the charge on the copper ion is +1 or +2, use
x to represent this unknown value. Remember that the charge on all the ions
must total zero. Each bromide ion has a charge of –1, so the total charge of
bromide ions is 2(–1).
CuBr2
x + 2(–1) = 0
x
= +2
The ionic charge of copper in this compound is +2.
Step 2 Write the name of the compound. Remember that Roman numerals are
included only if the metal has more than one ionic charge.
The name of CuBr2 is copper(II) bromide.
Practice
What is the chemical name of PbO2?
Whenever you have to write the chemical name of a compound containing
a metal, first check whether the metal has more than one ionic charge.
T RY THIS
TWO SHADES OF IRON
SKILLS: Observing, Analyzing
In this activity, you will compare the properties of the two ionic
forms of iron. To do this, you will use two different compounds of
iron: iron(III) chloride and iron(II) sulfate. The chloride and sulfate
ions arecompressed
colourless and dodangerously
not react with oxygen. This means
reactive
material
that thegasiron ions alone are
responsible
for all observable colour
differences/changes.
flammable and
combustible
biohazardous
infectious material
material and Materials: eye protection; lab apron; 100 mL
Equipment
poisonous
and
graduated cylinder; 250 mL
Erlenmeyer
flask; laboratory scoop;
infectious material
oxidizing
causing
immediate
scale; weighing
paper;
500
mL
Erlenmeyer
flask containing
material
and serious toxic
iron(III) chloride solution;effects
distilled water; 1.0 g iron(II) sulfate
poisonous and
Both
arematerial
irritants. Wash any spills on the
corrosiveiron compounds
infectious
material
causing other
skin, in the eyes, ortoxiconeffects
clothing immediately with plenty of
cold water. Report any spills to your teacher.
1. Put on your eye protection and lab apron.
SKILLS HANDBOOK
1.B., 3.B.
3. Add about 100 mL of distilled water to the 250 mL flask.
4. Add the 1.0 g of iron(II) sulfate to the water in the flask.
5. Swirl to dissolve the solid. Record your observations.
6. Continue swirling gently for about 1 min. Compare the colour
of your solution with that of the iron(III) chloride solution.
7. Follow your teacher’s instructions for the disposal of the
solutions. Clean up your work area and wash your hands.
A. Swirling the solution allows oxygen from the atmosphere
to enter the solution and react with the chemicals in the
solution. What evidence of a chemical change did you
observe? K/U
B. Suggest a possible explanation for the evidence you
observed. C
2. Observe the solution of iron(III) chloride that your teacher has
prepared. Record your observations.
NEL
5.7 Names and Formulas of Ionic Compounds
199
IN
SUMMARY
• When writing chemical formulas of ionic
compounds, the number of each ion must
balance the positive and negative charges
so that the overall charge is zero.
• The numbers of the ions in the chemical formula
of an ionic compound must be in the simplest
ratio. The subscript “1” is not included in a
chemical formula.
CHECK
• Ionic compounds are named with the metal
first and then the non-metal. The ending of the
non-metal’s name is changed to “ide.”
• Some metals can form ions with different
charges. In the name of an ionic compound
containing one of these metals, the charge is
written as Roman numerals in brackets after the
name of the metal.
YOUR LEARNING
1. (a) Describe at least one idea or skill in the reading that
you think you will need to practise.
9. Copy and complete Table 4. K/U
Table 4 Chemical Names and Formulas of Ionic Compounds
(b) How do you plan to improve with this idea or skill?
(c) Discuss your plans with your teacher.
2. Name each of these compounds: K/U
(a) CaF2
(b) K2S
(c) Al2O3
(d) LiBr
(e) Ca3P2
3. Determine the chemical formula of the ionic compound
that forms when each of these pairs of elements react. K/U
(a) K and Br
(b) Ca and O
(c) Na and S
4. Which is the correct chemical formula of tin(IV) oxide:
SnO2 or Sn2O4? Why? K/U
5. Copper forms two different compounds with bromine. One
compound contains one copper ion for every bromide ion.
The other contains two bromide ions for every copper
ion. Name and write the chemical formulas of these two
compounds. K/U
6. Why must the net charge on any ionic compound always
equal zero? K/U
7. Write the chemical formulas for the following
compounds: K/U
(a) calcium chloride
(b) aluminum bromide
(c) magnesium sulfide
(d) lithium nitride
(e) calcium nitride
8. The blue compound shown in Figure 1(a) was traditionally
called blue vitriol or bluestone. The systematic name for
this compound is copper(II) sulfate. What is the advantage
of a systematic naming system for chemicals? A
200
Name
Formula
C
Chapter 5 • Chemicals and Their Properties
(a)
iron(II) bromide
(b)
manganese(IV) oxide
(c)
tin(IV) chloride
(d)
copper(I) sulfide
(e)
iron(III) nitride
(f)
copper(II) oxide
(g)
PbCl2
(h)
Fe2O3
(i)
SnS
(j)
Cu3P2
(k)
CaBr2
(l)
CuF2
(m)
K3P
(n)
Cu3P
10. Magnetite is a substance composed of only iron
ions and oxygen ions. It has the chemical formula Fe3O4
(Figure 4). Given that iron has only two common ionic
charges, suggest an explanation for this formula. T/I
Figure 4
NEL