What is the oxidation number of carbon in carbon dioxide, CO2?

The oxidation number of an atom in a molecule is the charge that it would have if all the bonds were ionic. In CO2, each oxygen atom is double-bonded to the carbon atom.

Step 1: Oxygen is more electronegative than carbon, so we assign the electrons in each C=O bond to the oxygen.

Step 2: Oxygen normally has an oxidation state of -2 in compounds. Since there are two oxygen atoms, the total charge contributed by oxygen is -4.

Step 3: The total charge of a neutral molecule like CO2 is zero. Therefore, to balance the -4 charge from the oxygen atoms, the oxidation state of the carbon atom must be +4.

So, the oxidation number of carbon in carbon dioxide, CO2, is +4.