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

Explanation

Transition metal ions generally possess one or more unpaired electrons. When visible light falls on a transition metal compound or ion, the unpaired electrons present in the lower energy d-orbital get promoted to high energy d-orbitals, called d-d transition, due to the absorption of visible light. Since, the energy involved in d-d transition is quantised, only a definite wavelength gets absorbed, remaining wavelengths present in the visible region gets transmitted. Therefore, transmitted light shows some colour complementary to the absorbed colour.