Which chlorine oxyanion has the greatest number of oxygen atoms?

The chlorine oxyanion with the greatest number of oxygen atoms is perchlorate ($Cl{O}_4^{-}$), which contains four oxygen atoms.12

In the series of chlorine-containing oxyanions, the naming convention uses prefixes and suffixes to indicate the number of oxygen atoms present:2

• Hypochlorite ($Cl{O}^{-}$): 1 oxygen atom12
• Chlorite ($Cl{O}_2^{-}$): 2 oxygen atoms12
• Chlorate ($Cl{O}_3^{-}$): 3 oxygen atoms12
• Perchlorate ($Cl{O}_4^{-}$): 4 oxygen atoms12

The prefix per- is used to designate the member of the series with the largest number of oxygen atoms.2