5.9
Polyatomic Ions
sodium nitrite
preservative and
colour enhancer
sodium phosphate
binding agent
sodium erythrobate
preservative
Figure 1 Hot dogs contain many
chemical additives.
3–
O
O
P
O
O
Figure 2 The phosphate ion is made
up of four oxygen atoms bonded to a
central phosphorus atom.
polyatomic ion an ion made up of more
than one atom that acts as a single
particle
C05-F29-UBOS10SB.ai
DID YOU KNOW?
Not Just for Baking
Sodium hydrogen carbonate (baking
soda) is an extremely versatile
substance. It can substitute for
toothpaste if you run out, it soaks
up odours from your shoes and
refrigerator, it is a good abrasive
cleaner, and it is even useful to help
relieve insect bites and sunburn.
GO TO NELSON SCIENCE
202
Processed foods contain a lot of sodium, mostly from sodium chloride, NaCl
(table salt). Sodium chloride enhances the flavour and extends the shelf-life of
food.
Other additives in processed foods also contribute to your daily sodium
intake (Figure 1). For example, sodium phosphate helps bind together the
meat in a hot dog. Sodium nitrite acts as a preservative and flavour enhancer
and gives hot dogs their pinkish colour. There is some concern about adding
compounds containing nitrite ions to foods. Nitrite ions react with substances
in the digestive tract to form nitrosamines in the body. Nitrosamines are
chemicals that have been linked to certain types of cancer in laboratory
animals. An occasional hot dog is unlikely to make you ill, but a steady diet
of processed meats is not recommended.
Nitrites are not always bad news, however. Research suggests that sodium
nitrite may help protect heart tissue after a heart attack or transplant. Also,
nitrites and a related group of compounds called nitrates are important plant
nutrients. They occur naturally in soil and are manufactured and spread
as fertilizer on many crops. You can see how difficult it is to make a simple
judgment about whether a chemical is good or bad.
The sodium compounds in Figure 1 are ionic compounds similar to the
others you have learned about in this chapter. Sodium phosphate, for example,
is a white solid, relatively stable, and an electrolyte. The chemical formula
for this compound is Na3PO4. Its cation is sodium, but its anion, phosphate,
(PO4)3–, is an example of a polyatomic ion (Figure 2). A polyatomic ion is an
ion that consists of a stable group of several atoms acting together as a single
charged particle. The ionic charge of a polyatomic ion is shared over the entire
ion rather than being on just one atom.
Table 1 lists some of the most common polyatomic ions and their ionic
charges. Note that all the ions are anions except ammonium. Also, note that
all the anion names end in “ate” except nitrite and hydroxide.
Table 1 Formulas and Charges of Common Polyatomic Ions
Name of polyatomic ion
Ion formula
Ionic charge
nitrate ion
NO3–
−1
nitrite ion
NO2–
−1
hydroxide ion
OH–
−1
hydrogen carbonate ion
(also called bicarbonate ion)
HCO3–
−1
chlorate ion
ClO3–
−1
carbonate ion
CO32–
−2
sulfate ion
SO42–
−2
phosphate ion
PO43–
−3
ammonium
NH4+
+1
Chapter 5 • Chemicals and Their Properties
NEL
Naming Compounds Involving
Polyatomic Ions
The strategy for naming polyatomic compounds uses the same steps that
you learned in the previous section. The only difference is that the anion
is named according to the polyatomic ion rather than the names of the
individual elements.
SAMPLE PROBLEM 1
Naming Compounds Involving
Polyatomic Ions
Write the name of the compound Na2CO3. (Sodium does not form multiple ions.)
Step 1 Write the name of the metal and check whether it has more than one ionic
charge. Sodium always has a charge of +1.
Step 2 Write the name of the compound.
The name of Na2CO3 is sodium carbonate.
Practice
What is the name of Ca(OH)2?
As the Practice example indicates, several polyatomic ions can be
associated with each cation. This is indicated by placing brackets around
the polyatomic ion and writing the subscript outside the brackets.
Some metal ions have more than one charge. Check this before naming
the compound. In compounds containing these ions, use a Roman numeral
to indicate the charge.
SAMPLE PROBLEM 2 Naming Compounds Involving
Polyatomic Ions
Write the name of the compound Fe(NO3)3.
Step 1 Write the name of the metal and check whether it has more than one possible
ionic charge. If it does, proceed to step 2. If not, proceed to step 3.
Iron has two possible ionic charges: +2 and +3.
Step 2 Determine the ionic charge of the metal.
Fe (NO3)3
x + 3 (−1) = 0
x = +3
The ionic charge of iron in this compound is +3, indicated by the Roman
numeral “III.”
Step 3 Write the name of the compound, with the Roman numeral if necessary.
The name of Fe(NO3)3 is iron(III) nitrate.
Practice
What is the name of CuSO4?
NEL
5.9 Polyatomic Ions
203
Writing Formulas Involving Polyatomic Ions
You can apply the rules that you have already learned for writing ionic
compound formulas to polyatomic compounds. Remember to treat the
polyatomic ion as one unit. Consider the following Sample Problem.
SAMPLE PROBLEM 3
Writing Formulas for Compounds
Containing Polyatomic Ions
What is the chemical formula of sodium phosphate?
DID YOU KNOW?
Step 1 Write the symbols of the each ion, beginning with the cation (metal).
The Phosphate Fiasco
In the 1960s, laundry detergents
included phosphates. The phosphateladen wastewater was flushed into
lakes and streams. Phosphates are
an important plant nutrient. The
increased dose of nutrients caused
unnaturally rapid growth in aquatic
plant populations. As these plants died
and decomposed, much of the oxygen
in the water was used up, killing many
aquatic animals. To avoid the problem,
chemical engineers redesigned
detergents to work effectively without
phosphates.
Na+
3–
O
Na+
O
P
O
O
Na+
Figure 3 Sodium phosphate, Na3PO4,
consists of three sodium ions and
one phosphate ion, PO43−. When this
compound dissolves, these four ions
separate. However, the phosphate ion
remains intact.
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Na
PO4
Step 2 Write the ionic charges above each ion.
+1
Na
−3
PO4
Step 3 Determine how many ions of each type are required to give a total charge of zero.
Total ionic charge:
3(+1)
Na
+
1(−3) = 0
PO4
Three sodium ions balance the −3 charge of the phosphate ion.
Step 4 Write the chemical formula using the coefficients as subscripts.
Na3PO4
The formula of sodium phosphate is Na3PO4.
Note that sodium phosphate contains three sodium ions and one
phosphate ion (Figure 3).
The “crisscross method” also works for compounds with polyatomic
ions. Sample Problem 4 shows how. Note that, if a coefficient is required for
the polyatomic ion, first put curved brackets around it and then write the
subscript outside of the final bracket.
SAMPLE PROBLEM 4
Writing Formulas for Compounds
Containing Polyatomic Ions
What is the chemical formula of copper(II) nitrate?
Step 1 Write the symbol of each ion, with its charge.
Cu2+
(NO3)–
Step 2 Crisscross the numbers of the ionic charges so that they now become
subscripts.
(NO3)–
Cu2+
Step 3 Write any necessary subscripts after the brackets around each ion. (Remember,
you do not need to write “1” as a subscript, so no brackets are necessary.)
Cu
(NO3)2
The formula of copper(II) nitrate is Cu(NO3)2.
Practice
What is the chemical formula of ammonium carbonate?
204
Chapter 5 • Chemicals and Their Properties
NEL
IN
SUMMARY
• Polyatomic ions are made up of more than one
atom, with a charge spread over the entire ion.
• Polyatomic ions are present in many natural
and artificial compounds. They are used as
food additives, fertilizers, and cleaners.
CHECK
YOUR LEARNING
1. Name the polyatomic ion in each of the following
compounds and name the compound. (Watch for metals
with more than one possible ionic charge.) K/U
(a) KNO3 (found in gun powder)
(b) Ca(OH)2 (an ingredient in plaster)
(c) CaCO3 (in chalk, limestone, and antacid medicines)
6. Write the chemical formulas of the following compounds.
Note that some of them contain polyatomic ions. K/U
(a) calcium sulfate
(e) calcium chlorate
(b) ammonium chloride
(f) tin(II) hydroxide
(c) copper(I) carbonate
(g) iron(IV) phosphate
(d) barium sulfide
(h) aluminum nitride
7. Explain why the chemical formula for calcium hydroxide,
Ca(OH)2, is not written as CaO2H2. K/U
(d) CuSO4 (a fungicide)
(e) KOH (used to make soap)
8. Most ionic compounds are made up of a metal cation
and a non-metal anion. Look at Table 1 on page 202 and
identify an exception to this rule. K/U
(f) Fe(NO3)3 (used in water treatment)
(g) Cu(ClO3)2 (used to colour fireworks)
(h) (NH4)3PO4 (an ingredient in bread dough)
2. Write the chemical formula for each of the following
compounds: K/U
(a) potassium nitrate (used to colour fireworks purple)
(b) barium sulfate (given prior to an X-ray of the intestine)
(c) ammonium nitrate (a common ingredient in fertilizer)
9. When writing the chemical formula of an ionic compound,
which ion is always written first? K/U
10. Copy Table 2 into your notebook. Complete your table
using the example provided as a guide. K/U
Table 2 Identifying Ions
(d) aluminum sulfate (used in preparing pickles)
Compound
(e) potassium chlorate (an explosive)
Fe(OH)3
(f) copper(II) nitrate (used to colour ceramics)
Cu(NO3)2
(g) lead(II) sulfate (found in car batteries)
Al2(SO4)3
(h) tin(II) phosphate (used in the dyeing of silk)
(NH4)2CO3
3. What is the most common ending for the name of
(a) a polyatomic anion?
(b) an anion made up of only one element?
K/U
4. Well water in agricultural areas is often monitored for
nitrate ions because of possible health effects. Where can
nitrate contamination come from on a farm? A
5. Write the names of the following compounds. Note that
some of them contain polyatomic ions. K/U
(a) SnCO3
(e) K2S
(b) CaCl2
(f) (NH4)2SO4
(c) Fe(OH)3
(g) Mn(ClO3)2
(d) MnO2
NEL
• Consider the polyatomic ion as a single unit
when writing the chemical formula for a
compound that includes a polyatomic ion. If
the compound contains more than one of the
polyatomic ion, write curved brackets around the
ion and write a subscript outside the brackets.
Cation(s)
Anion(s)
1 Fe3+
3 OH–
K3PO4
11. The names “sodium chloride” and “sodium chlorate” sound
similar. However, these compounds are very different.
Sodium chloride enhances the flavour of food while sodium
chlorate is a toxic herbicide. For each compound, write
(a) the chemical formula
(b) the chemical formula of the anion
(c) the chemical formula of a compound that this anion
makes with calcium K/U
12. Describe one strategy you can use to reduce the amount
of salt in your diet. A
(h) PbI2
5.9 Polyatomic Ions
205