WHAT is the formal charge of CO₃ ??

The carbonate ion [CO₃^2-] has an overall charge of -2. In the carbonate ion, the carbon atom is bonded with a double bond to an oxygen atom, and with single bonds to two oxygen atoms.

The formal charge on each of the atoms can be calculated as follows

Formal charge on an atom in a lewis structure =  total number of valence electrons in free atom - (total number of non-bonding electrons) - 1/2 (total number of bonding electrons)

Now formal charge on carbon = 4 - 0 - 1/2 (8) = 0

Formal charge on oxygen bonded with the double bond = 6 - 4 - 1/2 (4) = 0

Formal charge on oxygen atoms bonded with single bond = 6 - 6 - 1/2 (2) = -1