The chemical formula of a carbonate ion is CO3^2-. It is an AX3 system which means it take the trigonal planar shape. There would be two single bonds of C-O and one double bond C-O in order for C to agree with the octet rule. Two minus charges are equally distributed to the three oxygen atoms so that each oxygen of the carbonate ion would have a charge of negative 2/3.
(a) How much negative charge is on each oxygen of the carbonate ion?