Polyvalent ion refers to when, the atom in an ion have a valency of more than one. i.e when the valency of the ion is greater than one. For example: O2− ion is a polyvalent ion as it has a valency of more than one, but it is a monoatomic ion. A polyatomic ion is the one which is formed of two or more atoms, bonded to each other by a covalent bond. For example; NH4, this is ammonium ion, it is a polyatomic ion as has more than one atom i.e, of nitrogen and hydrogen , but it is not polyvalent as its charge is only one.