Explain why:
(i) Transition elements form coloured compounds. (ii) `Cu^(+)` is diamagnetic but `Cu^(2+)` is paramagnetic (Z=29)

Explanation

Formation of coloured ions of transition elements can be attributed to the ability of electrons to make a transition from lower to higher energy d-orbitals after absorbing energy from visible light i.e., they undergo d-d transitions. The energy of excitation corresponds to the frequency of light absorbed, this frequency lies generally in visible region hence making the ions coloured.