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.

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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.