Which chlorine oxyanion has the fewest oxygen atoms?

The chlorine oxyanion with the fewest oxygen atoms is the hypochlorite ion, represented by the chemical formula ${\text{ClO}}^{-}$.12

In the series of chlorine-containing oxyanions, the prefix hypo- (meaning "less than") is used to designate the member with the smallest number of oxygen atoms. The complete series of these ions, ordered by increasing oxygen content, is as follows:2

• Hypochlorite (${\text{ClO}}^{-}$)12
• Chlorite (${\text{ClO}}_2^{-}$)12
• Chlorate (${\text{ClO}}_3^{-}$)12
• Perchlorate (${\text{ClO}}_4^{-}$)12