What is the oxidation number of copper?
The oxidation number of copper depends on its state.
The oxidation number of metallic copper is zero.
ON = +2: Examples are CuCl₂, CuO, and CuSO₄. See, for example
ON = +1: Examples are CuCl, Cu₂O, and Cu₂S.
ON = +3: Examples are KCuO₂ and K₃CuF₆.
ON = +4: An example is Cs₂CuF₆.
Hope this helps.