What is a Polyatomic Ion? (Definition & Examples)

A Polyatomic Ion (Poly = many, Atomic = atoms) is a tightly bound group of two or more different atoms that carry a net electrical charge (either positive or negative) and behave as a single, indivisible unit during chemical reactions.

• The atoms inside the group are held together by strong covalent bonds.
• The net charge on the ion belongs to the entire group as a whole, not just one specific atom inside it.

• Sulphate Ion ($SO_4^{2-}$): Made of one Sulphur and four Oxygen atoms. It carries a net charge of -2. It is found in Sulphuric Acid and Copper Sulphate.
• Nitrate Ion ($NO_3^{-}$): Made of one Nitrogen and three Oxygen atoms. It carries a net charge of -1.
• Hydroxide Ion ($OH^{-}$): Made of one Oxygen and one Hydrogen atom. It carries a charge of -1. It is the defining feature of strong bases like Sodium Hydroxide (NaOH).
• Carbonate Ion ($CO_3^{2-}$): Found in chalk and marble (Calcium Carbonate).

Almost all common polyatomic ions are negatively charged (anions). However, there is one very famous positive polyatomic ion (cation):

• Ammonium Ion ($NH_4^{+}$): Made of one Nitrogen and four Hydrogen atoms. It carries a net positive charge of +1.