Clusters of atoms that act as an ion are called polyatomic ions. They carry a fixed charge on them. These ions contain more than one atoms which may be same or of different kinds and behave as a single unit. For example, ammonium ion, NH₄⁺, is a polyatomic ion which is made up of two types of atoms, nitrogen (N) and hydrogen (H) joined together. Similarly, carbonate - CO₃^-2 sulphate - SO₄^-2, nitrate - NO₃^- and hydroxide ions OH^- are all polyatomic ions.