The reaction 2 Cu + O2 → 2 CuO has long been called an oxidation-reaction. Copper also reacts with chlorine:
Cu + Cl2 → CuCl2. In such a reaction:
A. Copper is also oxidized even though no oxygen is present.
This is because if it gains electrons, it is reduced - however, you can see that it lost an electron, while chlorine gained one, which means that this is oxidization.
