Polyatomic Ion
A covalently bonded group of atoms that carries an overall electrical charge and behaves as a single unit in reactions.
Unit
C (charge)
In depth
Also called a radical in older texts, a polyatomic ion keeps its identity through dissolution and most reactions, so it is written as an unbroken group in formulae and enclosed in brackets when subscripted. Their charges are delocalised over the whole group, which is why many are resonance-stabilised.
Examples in the real world
Sulfate SO₄²⁻, nitrate NO₃⁻, carbonate CO₃²⁻, phosphate PO₄³⁻, hydroxide OH⁻ and ammonium NH₄⁺.