Balanced chemical equation: C(s) + O₂(g) → CO₂(g).
Oxidation half reaction: C⁰ → C⁺⁴ + 4e⁻.
Reduction half reaction: O₂⁰ + 4e⁻ → 2O⁻².
Carbon is oxidized, it changes oxidation number from zero (in carbon element) to oxidation number +4 (in carbon dioxide).
Oxygen is reduced, it changes oxidation number from zero (in molecule of oxygen) to oxidation number -2 (in carbon dioxide).
