selected polyatomic ion

$$CrO_4^{2-}$$

chromate

$$Cr_2O_7^{2-}$$

dichromate

$$MnO_4^-$$

permanganate

$$NO_2^-$$

nitrite

$$NO_3^-$$

nitrate

$$O_2^{2-}$$

peroxide

$$OH^-$$

hydroxide

$$PO_4^{3-}$$

phosphate

$$SCN^-$$

thiocyanate

$$SO_3^{2-}$$

sulfite

$$SO_4^{2-}$$

sulfate

$$HSO_4^-$$

hydrogen sulfate

$$S_2O_3^{2-}$$

thiosulfate

hydronium

$$H_3O^+$$

mercury(I)

$$Hg_2^{2+}$$

ammonium

$$NH_4^+$$

acetate

$C_2H_2O_2$ and $CH_3COO^-$

cyanide

$$CN^-$$

carbonate

$$CO_3^{2-}$$

hydrogen carbonate

$$HCO_3$$

oxalate

$$C_2O4^{2-}$$

hypochlorite

$$ClO^{-}$$

chlorite

$$ClO_2^{-}$$

chlorate

$$ClO_3^-$$

perchlorate

$$ClO_4^-$$