Writing the electronic configuration of Cu : [Ar] 4s13d10, I feel that the number of valence electrons in a copper atom is 11. But it doesn't make any sense to me why copper has a valence of 1 or 2 as in case of CuSO4 .... which is 2. Going the usual way, if we consider Sodium, which has an atomic number of 11, we find that it's electronic configuration is [Ne]3s1 and it's very clear why it has a valency of 1. But why doesn't copper go that way ? Thanks for any help.