Copper is a transition metal, with unfilled d
orbitals.
Ground state electron configuration: [Ar].3d10.4s1
Shell structure: 2.8.18.1
Transition
metals may exhibit multiple oxidation states
(+1, +2, +3, etcÉ).
These are not easily predicted by position in the periodic
table.
Transition metals
ions in aqueous solutions frequently are brightly colored, also due to d orbitals (e.g. Cu ions are blue).