Ions are negatively or positively charged particles.

Ions can be monoatomic, for example, Ti²⁺, Rb⁺, Al³⁺, N^3-, S^2- and Br^-, or polyatomic, for example, CH₃COO^-, O₂^2-, PO₄^3-, SO₃^2-, NO₃^-, ClO₂^-, NH₄⁺ and OH^-.