Valency is the number of electrons an atom can gain, lose or share in order to attain a stable electron configuration. It is also known as oxidation state.

The valency of HCO3^- (bicarbonate) is -1.
The valency of HSO4^- (bisulfate) is -1.
The valency of H2PO4^- (dihydrogen phosphate) is -1.
The valency of CO3^2- (carbonate) is -2.

It's important to note that in polyatomic ions, the overall charge of the ion determines the valency.