Grouping atoms to form polyatomic ions

Ions can be polyatomic, composed of a group of atoms. For example, take a look at Table 5-3 in the preceding section. Notice anything about the mercury (I) ion? Its ion symbol,

Hg₂²⁺, shows that two mercury atoms are bonded together. This group has a 2+ charge, with each mercury cation having a 1+ charge. The mercurous ion is classified as a polyatomic ion. Similarly, the symbol for the sulfate ion, SO₄^2–, indicates that one sulfur atom and four oxygen atoms are bonded together and that the whole polyatomic ion has two extra electrons: a 2– charge.

Polyatomic ions are treated the same as monoatomic ions (see “Naming ionic compounds,” later in this chapter). Table 5-4 lists some important polyatomic ions.