Q.Write the general electronic configuration of lanthanoids. Why are most of the compounds of transition metals coloured?

Lanthanoids fill the 4f subshell; colour in transition-metal compounds arises from d–d transitions.

General electronic configuration of lanthanoids: $[Xe]\,4f^{1-14}\,5d^{0-1}\,6s^2$ — the general trend is progressive filling of the inner 4f orbitals (from La to Lu) while the outer $6s^2$ (and occasionally one $5d$) electrons remain largely constant.

Why most transition-metal compounds are coloured: Transition metal ions typically have partially filled d-orbitals. In the presence of ligands, the degenerate d-orbitals split into two sets of different energies (crystal field splitting). An electron can absorb a photon of visible light and get promoted from the lower-energy set of d-orbitals to the higher-energy set (a d–d transition). Since only part of the visible spectrum is absorbed, the complementary colour i …