SO3 wont react with molecular oxygen because in SO3 sulphurs oxidation state is +6 which is the maximum oxidation number possible for sulphur, hence it can\'t react with oxygen and expand its oxidation state. Solution SO3 wont react with molecular oxygen because in SO3 sulphurs oxidation state is +6 which is the maximum oxidation number possible for sulphur, hence it can\'t react with oxygen and expand its oxidation state..