Question

Why Cu shows +1,+2 and +3 oxidation states while it has no unpaired electrons in 3d orbitals?

Answer 1

in gaseous phase Cu shows +1 because it is more stable while in solid state +2 is more stable

Answer 2

Atomic copper has the electron configuration [Ar] 3d10 4s1[Ar] 3dX10 4sX1. By removing one electron and producing Cu+1CuX+1, an inert gas configuration [Ar] 3d10 4s0[Ar] 3dX10 4sX0 is produced. While it does take a lot more energy to remove the second electron from copper. if this energy can be offset by the energy gained through bond formation and lattice energy (or hydration energy) then Cu+2CuX+2 compounds will form. In fact Cu+2CuX+2 is the most common oxidation state of copper so the energetics must generally work out that the energy gained by forming more than one bond to copper and gaining additional lattice stabilization, more than offsets the energy cost of removing that second electron. Removal of additional electrons can also occur with copper to form Cu+3CuX+3 and Cu+4CuX+4 compounds,