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.
