<P> Copper (I) oxide may be produced by several methods . Most straightforwardly, it arises via the oxidation of copper metal: </P> <Dl> <Dd> 4 Cu + O → 2 Cu O </Dd> </Dl> <Dd> 4 Cu + O → 2 Cu O </Dd> <P> Additives such as water and acids affect the rate of this process as well as the further oxidation to copper (II) oxides . It is also produced commercially by reduction of copper (II) solutions with sulfur dioxide . Aqueous cuprous chloride solutions react with base to give the same material . In all cases, the color is highly sensitive to the procedural details . </P>

What is the oxidation number of the copper atom in cu2o(s)