Compound ion is an ion which consists of two or more atoms of

different elements to form a single unit.

Ex: $S{O}_4^{-2}$

It behaves as single unit and is composed of one atom S, and four

atoms of Oxygen.

Ex: $N{O}_3^{-}$

One atom of Nitrogen and three atoms of Oxygen, it behaves as

single unit.

Ex: $P{O}_4^{-3}$

One atom of Phosphorus and four Oxygen atom, it behaves as single

unit.