How to find the valency of a polyatomic ion?

The valencies of the constituent atoms are combined by the poly atomic ion, carbonate, has a formula of CO3 the valence state it is in of oxygen ions is - 2 the periodic table group it's in Group 16 the two to the left of Group 18, the noble gases: 16-18 = - 2) and their are 3 oxygen atoms in the poly atomic ion, so the oxygen atoms contribute - 6 to the valency of the poly atomic ion. The one carbon atom has a valency of 4+. The overall charge on the carbonate ion is - 6 + 4 = - 2. So carbonate has a charge of - 2.