What gas in the air has reacted with the cooper to form CuO?

The gas in the air that reacts with copper (Cu) to form copper(II) oxide (CuO) is oxygen (O₂). When copper is exposed to oxygen, it undergoes an oxidation reaction, resulting in the formation of copper(II) oxide, a black solid. This reaction is part of the broader process of corrosion, where metals react with environmental elements.