Question

Explain why most of the tripositive ions of lanthanoids are coloured.

Answer 1

1. Most tripositive lanthanoid ions have unpaired electrons in the 4f orbitals.
2. The colour arises from absorption of visible light due to electronic transitions within these 4f orbitals, called f-f transitions.
3. The energy needed for these f-f transitions varies for different lanthanoid ions, giving rise to different colours.
4. The 4f orbitals are shielded and located inside the atom, so these transitions are less affected by the environment, leading to characteristic colours.
5. Lanthanoid ions with empty (f⁰), half-filled (f⁷), or completely filled (f¹⁴) 4f subshells are colourless because no f-f transitions occur.
6. Some ions absorb light outside the visible range (UV or IR) and hence appear colourless.

Thus, the presence and nature of electronic transitions involving 4f electrons mainly cause the colours in lanthanoid ions.