Polyatomic ion

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A polyatomic ion is an ion consisting of multiple atoms associated together by covalent bonds which can be considered as acting as a single unit in the context of acid/base chemistry or in the formation of salts. Note that in this context, a polyatomic ion is also referred to as a radical.

Some commonly-occurring polyatomic ions and their charges are indicated below

carbonate    CO₃^2-

bicarbonate  HCO₃^-

nitrate      NO₃^-

phosphate    PO₄^3-

sulfate      SO₄^2-

acetate      CH₃C(=O)O^-

ammonium NH₄⁺

hydroxide    OH^-

pyridinium C₅H₅NH⁺

The atoms of a polyatomic ion are always associated with each other via covalent or coordinate-covalent bondss.

A polyatomic ion that is primarily associated with other atoms by covalent, rather than ionic, forces might more properly be referred to as a functional group.

Note that many of the common negatively-charged (anionic) polyatomic ions are oxides of non-metallic elements, and can be considered the conjugate bases of the corresponding acids.