Many ions are monoatomic, which means that they are composed of just one atom. However, ions can also be polyatomic, composed of a group of atoms.

For example, take a look at the following table. 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.

Polyatomic ions are treated the same as monoatomic ions. The following list shows names and symbols for some important polyatomic ions.

As shown above, the symbol for the sulfate ion is:

This indicates that one sulfur atom and four oxygen atoms are bonded together and that the whole polyatomic ion has two extra electrons.