Polyatomic ions are charged groups of atoms. An example is ammonium ion, NH₄⁺. It has five atoms (one nitrogen and four hydrogens) that share a charge of +1. The polyatomic ions remain intact, and parentheses may be required when using subscripts. For example, ammonium chloride is NH₄Cl and ammonium sulfide is (NH₄)₂S. Ammonium is the only polyatomic cation. Common anions are shown in Table 1.

There are many polyatomic anions. Many occur in families of names. Start by learning the polyatomic ions ending with “-ate” such as chlorate (ClO₃^–), nitrate (NO₃^—), sulfate (SO₄^2-), carbonate (CO₃^2-), and phosphate (PO₄^3-).

The corresponding “-ite” ion name has one less oxygen and the same charge. For example, chlorite ion is ClO₂-. Less commonly used names are the “per__-ate” and “hypo__-ite” forms to indicate different numbers of oxygen.

Key in on the chlorate family in Table 1 to construct names for other ions. For example, sulfite (not in the table) would be SO₃^2-, because it has the same charge and one less oxygen than sulfate (SO₄^2- in the table).