When you look at the electron configuration of neutral copper, it is 1s2 2s2 2p6 3s2 3p6 4s1 3d10. Assuming that the two most stable electronic states for an atom or ion mean a full outer shell or a half-full outer shell, neutral copper should be quite stable (1 half full and 1 full). Copper(I) should have the configuration ... 4s0 3d10, a full outer orbital. Copper(II) would have ...4s0 3d9, neither full nor half-full. How come it has quite a high stability, as in, for example, copper(II) sulphate, seeing that copper(I) sulphate does not exist?