Formation of coloured ions of of d-block elements:
Most of the transition metal compounds are coloured in their solid or solution form. The colour of transition metal ions is due to the presence of unpaired electrons in it and the energy gap between two energy levels in the same d-subshell being small. Hence very small amount of energy is required for excitation of electrons from one energy level to the other. The energy can be easily provided by the visible light. The colour observed corresponds to the complementary colour of the light absorbed.
The colour of some transition metal ions are given in the Table.
It may be noted that Zinc, Cadmium and Mercury salts do not form any coloured compounds because of the absence of vacant d orbitals to which electrons can be excited. Sc3+ ions are also colourless because of the absence of d-electrons.