Every element in the compound has its own formal charge. The formula is:
Formal charge = Valence electrons - Nonbonding electrons - Bonding electrons/2
Let the Lewis structure of the compound aid you.
1. For CO:
Formal charge of C = 4 - 2 - 6/2 = -1
Formal charge of O = 6 - 2 - 6/2 = +1
2. For CO₂:
Formal charge of C = 4 - 0 - 8/2 = 0
Formal charge of each O = 6 - 4 - 4/2 = 0
3. For CO₃²⁻:
Formal charge of C: 4 - 0 - 8/2 = 0
Formal charge of two O's single-bonded to C = 6 - 6 - 2/2 = -1
Formal charge of the O double-bonded to C = 6 - 4 - 4/2 = 0
