common polyatomic ions

acetate

$$C_2H_3O_2^{-}$$

carbonate

$$CO_3^{2-}$$

hydrogen carbonate (bicarbonate)

$$HCO_3^{-}$$

hydroxide

$$OH^{-}$$

nitrite

$$NO_2^{-}$$

nitrate

$$NO_3^{-}$$

chromate

$$CrO_{4^{}}^{2-}$$

dichromate

$$Cr_2O_7^{2-}$$

phosphate

$$PO_4^{3-}$$

hydrogen phosphate

$$HPO_4^{2-}$$

dihydrogen phosphate

$$H_2PO_4^{-}$$

ammonium

$$NH_4^{+}$$

hypochlorite

$$ClO^{-}$$

chlorite

$$ClO_2^{-}$$