There are two oxides of copper because it can bond with non-metals by donating either one or two electrons. Oxygen forms compounds with metals by gaining two electrons, so it can accept one electron from each of two copper atoms — forming copper(I) oxide — or two electrons from one atom — forming copper(II) oxide. The numbers “I” and “II” represent the number of electrons the metal has provided; this is known as its oxidation number. The chemical formulae of these compounds are Cu2O and CuO, respectively