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chapter
18
Chemical Bonds
3
section ●
Writing Formulas and Naming 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
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Binary Ionic Compounds
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 You’ll Learn
how to determine
oxidation numbers
■ how to write formulas
and names for ionic
compounds
■ how to write formulas
and names for covalent
compounds
■
Study Coach
Create a Quiz 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.
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.
C Build Vocabulary
●
Make two quarter-sheet
Foldables as shown below.
Record information about the
oxidation number of the
elements and rules for writing
formulas as you read.
Oxidation
Number:
Ionic
Compounds:
Reading Essentials
327
1
0
Hydrogen
Helium
1
H
2
3
4
3
2
1
2
He
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
11
Na
12
Mg
Potassium
Calcium
19
K
20
Ca
Rubidium Strontium
Gallium Germanium
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
Picture This
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.
What are special ions?
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
Picture This
2. Compare Circle the Roman
numerals and the oxidation
numbers for each element
in the table.
328
CHAPTER 18 Chemical Bonds
Name
Oxidation
Number
Copper(I)
1+
Copper(II)
2+
Iron(II)
2+
Iron(III)
3+
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Sodium Magnesium
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.
3. Infer What does the
formula CaF2 tell you about
the compound?
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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?
Applying Math
4. Calculate What is the
least common multiple of
5 and 2?
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
329
5. Restate What will a
compound be when its
positive and negative
charges balance?
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 Li3N. 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.
How do you name a binary ionic compound?
6. Apply Write the formula
for calcium chloride. Use
the periodic table to
identify the positive and
negative ions.
330
CHAPTER 18 Chemical Bonds
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 on 404. 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.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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 Group 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.
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,
write 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.
7. Determine What is the root
name of oxygen?
Table 4 Elements in Binary Compounds
Element
-ide
Name
Oxygen
Oxide
Phosphorus
Phosphide
Nitrogen
Nitride
Sulfur
Sulfide
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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 on
404? 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 poly- means “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.
8. Apply Write the name of
the compound FeI2.
Reading Essentials
331
Polyatomic Ions
Picture This
9. Explain Use the table to
determine what elements
make up acetate.
Charge
Name
Formula
1
ammonium
NH4
1
acetate
chlorate
hydroxide
nitrate
C2H3O2
CIO3
OH
NO3
2
carbonate
sulfate
CO32
SO42
3
phosphate
PO43
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.
Applying Math
10. Determine How many
2+ ions does it take to
balance the charge on one
4– ion?
332
CHAPTER 18 Chemical Bonds
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.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How do you name a polyatomic compound?
Balancing Negative Charges Now write the formula for the
negative ion. The table on 408 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
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.”
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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.
11. Analyze How many water
molecules does the hydrate
LiNO2 • H2O have?
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.
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.
12. Identify What is the
element N?
What is the compound NO?
Reading Essentials
333
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?
13. Use Numbers What does
the prefix tetra- mean?
334
CHAPTER 18 Chemical Bonds
Prefixes for
Covalent Compounds
Number
of Atoms
Prefix
1
mono-
2
di-
3
tri-
4
tetra-
5
penta-
6
hexa-
7
hepta-
8
octa-
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Picture This
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 monois 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.
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
1. Review 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
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
1.
2.
3.
4.
End of
Section
Reading Essentials
335