Question

Explain the magnetic behaviour exhibited by the tripositive lanthanoid ions.

Answer 1

1. Most tripositive lanthanoid ions are paramagnetic due to unpaired 4f electrons; La³⁺ and Lu³⁺ are exceptions and are diamagnetic as they have no unpaired electrons.
2. Their magnetic moments do not depend only on the number of unpaired electrons, unlike d-block elements.
3. This is because the 4f electrons are deeply buried and shielded by the outer 5s² and 5p⁶ electrons, making their orbital motion contribute significantly to magnetism.
4. Therefore, the magnetic moment arises from both electron spin and orbital angular momentum.
5. The resultant magnetic moment (μ_J) is given by μ_J = `g sqrt [J(J+1)]`, where J = L + S, incorporating both orbital (L) and spin (S) quantum numbers.
6. This combined effect causes anomalous magnetic moment values, which vary non-linearly with unpaired electron number, and explains differences from transition metal magnetism.