Copper is a transition metal with partially filled d orbitals.
A transition metal may exhibit multiple oxidation states, such as +1, +2,
or +3, oxidation states which are not
easily predicted by its position in the periodic table.
Because of the partially filled d orbitals, a transition metal ion in aqueous solution frequently is brightly colored, e.g.,
copper ions are blue in aqueous solution.
Zinc and aluminum
are both stronger reducing agents than copper, see their redox potentials, so
either will yield metallic copper from a
solution of a copper salt.