Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Nomenclature
Naming Compounds That Contain Polyatomic Ions

Objective

· To learn the names of common polyatomic ions and how to use them in naming compounds.

A type of ionic compound that we have not yet considered is exemplified by ammonium nitrate, , which contains the polyatomic ions and . As their name suggests, polyatomic ions are charged entities composed of several atoms bound together. Polyatomic ions are assigned special names that you must memorize to name the compounds containing them. The most important polyatomic ions and their names are listed in Table 5.4.

Table 5.4. Names of Common Polyatomic Ions

Note in Table 5.4 that several series of polyatomic anions exist that contain an atom of a given element and different numbers of oxygen atoms. These anions are called oxyanions . When there are two members in such a series, the name of the one with the smaller number of oxygen atoms ends in -ite, and the name of the one with the larger number ends in -ate. For example, is sulfite and is sulfate. When more than two oxyanions make up a series, hypo- (less than) and per- (more than) are used as prefixes to name the members of the series with the fewest and the most oxygen atoms, respectively. The best example involves the oxyanions containing chlorine:

Naming ionic compounds that contain polyatomic ions is very similar to naming binary ionic compounds. For example, the compound is called sodium hydroxide, because it contains the (sodium) cation and the (hydroxide) anion. To name these compounds, you must learn to recognize the common polyatomic ions. That is, you must learn the composition and charge of each of the ions in Table 5.4. Then when you see the formula , you should immediately recognize its two “parts”:

The correct name is ammonium acetate.

Remember that when a metal is present that forms more than one cation, a Roman numeral is required to specify the cation charge, just as in naming Type II binary ionic compounds. For example, the compound is called iron(II) sulfate, because it contains (to balance the charge on ). Note that to determine the charge on the iron cation, you must know that sulfate has a charge.

Interactive Example 5.7. Naming Compounds That Contain Polyatomic Ions

Give the systematic name of each of the following compounds.

a.

b.

c.

d.

e.

f.

Solution

Self-Check: Exercise 5.6

· Name each of the following compounds.

a.

b.

c.

d.

e.

f.

g.

See Problems 5.35 and 5.36.

Example 5.7 illustrates that when more than one polyatomic ion appears in a chemical formula, parentheses are used to enclose the ion and a subscript is written after the closing parenthesis. Other examples are and .

In naming chemical compounds, use the strategy summarized in Fig. 5.2. If the compound being considered is binary, use the procedure summarized in Fig. 5.1. If the compound has more than two elements, ask yourself whether it has any polyatomic ions. Use Table 5.4 to help you recognize these ions until you have committed them to memory. If a polyatomic ion is present, name the compound using procedures very similar to those for naming binary ionic compounds.

Figure 5.2.

Overall strategy for naming chemical compounds.

Interactive Example 5.8. Summary of Naming Binary Compounds and Compounds That Contain Polyatomic Ions

Name the following compounds.

a.

b.

c.

d.

e.

Solution

Self-Check: Exercise 5.7

· Name the following compounds.

a.

b.

c.

d.

e.

f.

g.

See Problems 5.29, 5.30, 5.31, 5.32, 5.33, 5.34, 5.35, and 5.36.