Final answer:
The oxidation state of carbon in CO3⁻² is +4 to satisfy the overall charge of -2 of the carbonate ion.
Step-by-step explanation:
To determine the oxidation state of carbon in CO₃⁻², one should consider the known oxidation states of the other atoms in the molecule and the total charge. Oxygen has a known oxidation state of -2. Given that CO₃⁻² has an overall charge of -2 and there are three oxygen atoms, the total contribution of the oxygen to the charge is 3(× -2) = -6. The overall charge of the molecule is -2, so the carbon must have an oxidation state that, when combined with -6 from the oxygens, gives an overall charge of -2. Hence, the oxidation state of carbon in CO₃⁻² must be +4, because +4 + (-6) = -2, which is the charge on the carbonate ion.