3 Writing Formulas and Naming
Compounds
6(D)
oxidation numbers
• how to write formulas and
names for ionic compounds
Before You Read
Shakespeare asked, “What’s in a name?” In this section, you are
going to learn what the names of chemical compounds can tell
you. On the lines below, explain what your name tells about you.
Read to Learn
• how to write formulas and
names for covalent
compounds
Focus
Binary Ionic Compounds
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What You’ll Learn
• how to determine
Alchemists were early scientists who tried to turn lead into
gold. They never succeeded, but they did develop some
laboratory methods and equipment that scientists still use today.
The alchemists also used symbols to write formulas.
The first formulas you will learn to write are for binary ionic
compounds. A binary compound is a compound made of two
elements. An example of a binary compound is potassium iodide, a
compound added to table salt.
What are oxidation numbers?
Before you can write the correct formula of a compound,
you need to know which elements combine to make that
compound. All elements in a certain group of the periodic table
have the same number of electrons in their outer energy levels.
So, elements in the same group all gain or lose the same number
of electrons.
Metals always lose electrons, and nonmetals always gain
electrons when they form ions. The oxidation number of an atom
tells you how many electrons an atom gains, loses, or shares to
become stable. It is the same as the charge on the ion.
The charge on the ion in an ionic compound is the same as
its oxidation number. For example, a sodium ion has a charge
of 1+ and an oxidation number of 1+. A chloride ion has a
charge of 1− and an oxidation number of 1−.
Reading Essentials • Chemical Bonds
329
As you read the text under each
heading, write a question that
your teacher might ask on a quiz.
Exchange your questions with
a partner and take each other’s
quizzes.
1+
Hydrogen
0
Helium
1
H
2+
3+
4+
3-
2-
1-
Lithium
Beryllium
Boron
Carbon
Nitrogen
Oxygen
Fluorine
Neon
3
Li
4
Be
5
B
6
C
7
N
8
O
9
F
10
Ne
Aluminum
Silicon
Phosphorus
Sulfur
Chlorine
Argon
13
Al
14
Si
15
P
16
S
17
Cl
18
Ar
Sodium Magnesium
11
Na
12
Mg
Potassium
Calcium
19
K
20
Ca
Gallium Germanium
Rubidium Strontium
2
He
Arsenic
Selenium
Bromine
Krypton
31
Ga
32
Ge
33
As
34
Se
35
Br
36
Kr
Indium
Tin
Antimony
Tellurium
Iodine
Xenon
37
Rb
38
Sr
49
In
50
Sn
51
Sb
52
Te
53
I
54
Xe
Cesium
Barium
Thallium
Lead
Bismuth
Polonium
Astatine
Radon
55
Cs
56
Ba
81
Tl
82
Pb
83
Bi
84
Po
85
At
86
Rn
Francium
Radium
87
Fr
88
Ra
Take a Look
1. Identify Highlight the
oxidation numbers on
the periodic table.
How are oxidation numbers related to the
periodic table?
Look at the periodic table above. Notice the numbers above
each column. These are the oxidation numbers for the elements
in the column. Notice how the oxidation numbers fit with the
periodic table groupings.
Part of the periodic table is not included in the figure above.
Some elements in this section can have more than one oxidation
number. The table shows some of these elements and their
oxidation numbers. Because these elements can have more than
one oxidation number, you must include more information when
you name them. When naming these compounds, the oxidation
number is expressed in the name as a roman numeral. For
example, when iron with an oxidation number of 3+ combines
with oxygen, the compound they form is iron(III) oxide.
Some Special Ions
Take a Look
2. Compare Circle the Roman
numerals and the oxidation
numbers for each element in
the table.
Name
Oxidation
Number
Name
Oxidation
Number
Copper(I)
1+
Chromium(II)
2+
Copper(II)
2+
Chromium(III)
3+
Iron(II)
2+
Lead(II)
2+
Iron(III)
3+
Lead(IV)
4+
Reading Essentials • Chemical Bonds
330
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How do transition elements form ions?
How many positive and negative ions must a
formula have?
Remember that ionic compounds are neutral. The ions in an
ionic compound have charges, but the compound itself does not.
The formula for an ionic compound must have the right number
of positive and negative ions to make the charges balance.
For example, sodium chloride has one sodium ion with a
charge of 1+ and one chloride ion with a charge of 1−. The 1+
balances the 1−. The correct formula for sodium chloride is
NaCl. The formula tells you that one positively charged ion
combines with one negatively charged ion and makes a neutral
compound.
What about the ionic compound calcium fluoride? A calcium
ion has a charge of 2+. A fluoride ion has a charge of 1-. The
charge 1- does not balance 2+. You need to have two fluoride
ions for every calcium ion to balance the charges. The formula for
the neutral compound calcium fluoride, CaF2, shows exactly that.
There is one calcium ion and two fluoride ions in the compound.
Think it Over
3. Infer What does the
formula CaF2 tell you about
the compound?
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How do you find the correct subscripts?
Sometimes you need to use math skills to write a formula
correctly. In the compound aluminum oxide, there are
aluminum ions and oxygen ions. But, how many of each one?
From the periodic table, you can get the information about
aluminum and oxygen that you need to write the formula.
By its place on the periodic table, you can tell that
aluminum has an oxidation number of 3+. That means the
aluminum ion has a charge of 3+. Oxygen has an oxidation
number of 2−, so its ion has a charge of 2−. You must find the
least common multiple of 3 and 2 to balance the charges. The
least common multiple of 3 and 2 is 6.
Multiply 3+ by 2 to equal 6+. You need two aluminum ions
to have a charge of 6+. Multiply 2− by 3 to equal 6−. You need
three oxygen ions to have a charge of 6-. The charges balance.
The correct formula for the neutral compound aluminum oxide
is Al2O3.
How do you write the formula of an ionic
compound?
You learned how to use the oxidation number to find the charge
on an ion. You also learned that the charges on the ions must
balance to form a neutral compound. Now you can write formulas
for ionic compounds. Follow rules 1, 2, and 3 on the next page.
Reading Essentials • Chemical Bonds
331
Apply Math
4. Calculate What is the
least common multiple of
5 and 2?
Think it Over
5. Restate What will a
compound be when its
positive and negative charges
balance?
6. Apply Write the formula
for calcium chloride. Use
the periodic table to
identify the positive and
negative ions.
How do you name a binary ionic compound?
When you know the formula, you can write the name of a
binary ionic compound by following these rules:
1. Write the name of the positive ion.
2. Look to see if the positive ion is listed in the Special Ions
table. If it is not in the table, go right to Step 3. If it is, the ion
can have more than one oxidation number. To find the
correct oxidation number, look at the formula of the
compound. The charge of the compound is always zero.
The negative ion can only have one possible charge. From the
table, pick the charge of the positive ion that balances
the negative charge. Write that positive ion’s symbol along
with the correct roman numeral in parentheses.
Reading Essentials • Chemical Bonds
332
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Think it Over
Writing Formulas You have learned how to find oxidation
numbers and their least common multiples. Now you can write
formulas for ionic compounds. What is the formula for an ionic
compound containing sodium and oxygen? Use these rules to
figure it out:
1. Write the symbol of the element that has the positive
oxidation number or charge. Sodium is a goup 1 element.
It has an oxidation number of 1+.
2. Write the symbol of the element with the negative oxidation
number or charge. Except for hydrogen, all nonmetals have
negative oxidation numbers. Oxygen has an oxidation
number of 2−.
3. The compound should be neutral. To make it neutral, the
positive charges have to balance the negative charges. It takes
two sodium ions to balance one oxygen ion. Thus, the
formula becomes Na2O.
Now use these rules to write the formula for lithium nitride.
Lithium and nitrogen are the two atoms that make this
compound. Look at the periodic table in the back of this book.
Lithium is in Group 1, so it forms ions with a 1+ charge. Write
the symbol for lithium, Li, first.
Find the oxidation number of nitrogen. Nitrogen is in group
15. It forms ions with a charge of 3−. You now can write LiN. Can
you stop now? Look at the charges of the two ions. Do 1+ and
3− balance? No, you cannot stop yet.
Use the number of the charge of a nitrogen ion as the
subscript for Li. Use the number of the charge of a lithium ion as
the subscript for N. That gives LiN. When an element has no
subscript, it means only one ion is in the compound. Do 3(1+)
and 3− balance? Yes, this is the correct formula.
3. Write the root name of the negative ion. The root is the first
part of the element’s name. For example, the root name of
chlorine is chlor-. The root name of oxygen is ox-.
4. Add the ending -ide to the root name. For example,
oxygen becomes oxide.
Do not use subscripts in the name of an ionic compound.
Subscripts can be used in the formula to figure out the charge on
a metal ion with more than one positive charge.
Think it Over
7. Determine What is the root
name of oxygen?
Elements in Binary
Compounds
Element
-ide Name
Oxygen
Oxide
Phosphorus
Phosphide
Nitrogen
Nitride
Sulfur
Sulfide
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How do you use these rules?
Use these rules to write the name of the compound CuCl. Find
the name of the positive ion on the periodic table. Cu is the symbol
for copper. Is copper in the Special Ions table? Yes, and the copper
ion can have a 1+ or a 2+ charge. To find the charge on the
copper in CuCl, look at the negative ion. Cl is the symbol for
chlorine. Chlorine is in group 17 of the periodic table. That means
it has an oxidation number of 1−. From the formula, you can see
that there is only one chloride ion in the compound. To balance a
1− charge on the chloride ion, the copper ion must have a 1+
charge. Now you can write copper(I) as the first part of the name.
Write the root name of the negative ion. The root name for
chlorine is chlor-. Add -ide to the root. That gives you chloride.
The correct name of CuCl is copper(I) chloride.
Compounds with Polyatomic Ions
Not all ionic compounds are binary compounds. The formula
for baking soda used in cooking is NaHCO3. Baking soda is an
ionic compound that is not binary. Compounds like baking soda
are made of more than two elements. These compounds have
polyatomic ions. A polyatomic ion is a charged group of atoms
that are bonded together by a covalent bond. The prefix polymeans “many” and polyatomic means “many atoms.” The
polyatomic ion in baking soda is the bicarbonate or hydrogen
carbonate ion. The symbol of this polyatomic ion is HCO3-.
Reading Essentials • Chemical Bonds
333
Think it Over
8. Apply Write the name of the
compound FeI2.
Polyatomic Ions
Take a Look
9. Explain Use the table to
determine what elements
make up acetate.
Charge
Name
Formula
1+
ammonium
NH4+
1-
acetate
chlorate
hydroxide
nitrate
C2H3O2CIO3OHNO3-
2-
carbonate
sulfate
CO32SO42-
3-
phospate
PO43-
How do you name a polyatomic compound?
Apply Math
10. Determine How many
2+ ions does it take to
balance the charge on one
4– ion?
How do you write the formula of a
polyatomic compound?
To write the formula for a compound with a polyatomic ion,
use the rules for writing the formula of a binary compound, but
add one more thing. If you need to show more than one
polyatomic ion, put parentheses around the formula for the ion
before you write the subscript.
Write the formula for barium chlorate. First, write the symbol
of the positive ion. The symbol for barium is Ba. Barium is in
group 2, so it forms a 2+ ion.
Reading Essentials • Chemical Bonds
334
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To write the name of a compound with a polyatomic ion,
first write the name of the positive ion. If the positive ion is
polyatomic, use the table above to find its name.
Next, write the name of the negative ion. Again, if it is
polyatomic, look up its name in the table. What is the name of the
compound K2SO4? K is the symbol of the positive ion. It is not
polyatomic. K is the symbol for potassium. The negative ion is
SO42–. It is polyatomic. Use the table again. K2SO4 is potassium
sulfate.
Now try naming the compound Sr(OH)2. You can see that the
positive ion is not polyatomic. Find the name for the symbol Sr
on the periodic table. It is strontium. The negative ion is
polyatomic. From the table of polyatomic ions, you can see that
OH– is named the hydroxide ion. The name of Sr(OH)2 is
strontium hydroxide.
Balancing Negative Charges Now write the formula for the
negative ion. The Polyatomic Ions table shows you the formula
for the chlorate ion is ClO3–. Are the charges on these two ions
balanced? No, 2+ does not balance 1−. It takes two chlorate ions
to balance the 2+ charge on the barium ion. The formula for
barium chlorate is Ba(ClO3)2.
Compounds with Added Water
Think it Over
Some ionic compounds have water molecules as part of their
structure. A hydrate is a compound that has water chemically
attached to its ions and written into its formula. The word hydrate
comes from a word that means “water.”
11. Analyze How many water
molecules does the hydrate
LiNO2 • H2O have?
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What are common hydrates?
When a solution of cobalt chloride evaporates, pink crystals
form. The crystals have six water molecules for each unit
of cobalt chloride. The formula for this compound is
CoCl2 • 6H2O. The compound’s name is cobalt chloride
hexahydrate. The prefix hexa- means “six,” so hexahydrate means
“six waters.”
You can remove water from these crystals by heating them. The
blue compound that results from heating contains no water. It is
called anhydrous, meaning “without water.” When anhydrous
CoCl2 is exposed to water, even water in the air, it absorbs water
and becomes pink again.
Plaster of paris becomes a hydrate when water is added. It
becomes calcium sulfate dihydrate, which is also known as
gypsum. The water added to the powder becomes part of the
compound.
Formulas for Hydrates To write a formula for a hydrate, first
write the formula for the compound. Then make a dot followed
by the number of water molecules. The dot stands for the ratio of
compound-to-water molecules. For example, the formula for
calcium sulfate dihydrate is CaSO4 • 2H2O, a compound of
calcium sulfate that contains two water molecules.
Think it Over
Naming Binary Covalent Compounds
Covalent compounds form between elements that are
nonmetals. Some nonmetals can form many compounds. For
example, nitrogen and oxygen can form N2O, NO, NO2, and
N2O5. Using the rules you learned earlier, all these compounds
would be called nitrogen oxide. Now you will learn how to give each
of these compounds a different name.
Reading Essentials • Chemical Bonds
335
12. Identify What is the
element N?
What is the compound NO?
How are prefixes used to name
covalent compounds?
The table below lists some Greek prefixes used to name covalent
compounds made with the same elements. These prefixes tell how
many atoms of each element are in a compound. For example, the
compound NO2 is nitrogen dioxide. The prefix di- tells you that
there are two oxygen atoms in the compound. N2O is dinitrogen
oxide. The compound has two nitrogen atoms.
The name of the compound N2O5 is dinitrogen pentoxide. This
name uses two prefixes. There are two nitrogen atoms, so dinitrogen
is used. There are also five oxygen atoms, so pentoxide is used.
What rules apply in naming covalent compounds?
Drop the last vowel of the prefix when the second element of
the compound begins with a vowel. In pentoxide, the a is dropped
from penta-.
There is a prefix to use when a compound has only one atom of
an element. The prefix is mono-. Many times mono- is not used.
Instead, it is understood that if no prefix is used, there is only one
atom of that element in a compound. In some cases, mono- is
used for emphasis. Carbon monoxide is one example.
Take a Look
13. Use Numbers What does
the prefix tetra- mean?
Prefixes for
Covalent Compounds
Prefix
1
mono-
2
di-
3
tri-
4
tetra-
5
penta-
6
hexa-
7
hepta-
8
octa-
Reading Essentials • Chemical Bonds
336
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Number
of Atoms
After You Read
Mini Glossary
binary compound: a compound made of two elements
hydrate: a compound that has water chemically attached to its ions
and written into its formula
oxidation number: the number that tells how many electrons an
atom gains, loses, or shares to become stable
polyatomic ion: a charged group of atoms that are bonded
together by a covalent bond
Review
1. Study the terms and definitions in the Mini Glossary. Write a sentence that
explains in your own words what a polyatomic ion is.
2. Complete the flow chart with the steps used in writing the name of a
binary ionic compound.
To write the name of a binary ionic compound
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1.
2.
3.
4.
Reading Essentials • Chemical Bonds
337