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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2021 · Paper I

Q8(b). Magnetic moments of lanthanide and first-row transition metal ions

Question

The observed magnetic moments of lanthanide ions in general differ from observed magnetic moments of first row transition metal ions. Explain by giving reason(s). Identify the lanthanide ions having magnetic moments corresponding to spin-only value, and those which are diamagnetic.

Answer in brief

In 3d ions the ligand field quenches the orbital moment, giving μ ≈ √(n(n + 2)) BM, while the buried 4f electrons of Ln³⁺ keep it, giving μ = gJ√(J(J + 1)) BM; Gd³⁺ shows the spin-only value and La³⁺ and Lu³⁺ are diamagnetic.

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Topic notes: lanthanides and actinides (lanthanide contraction, magnetic moments, separation), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverslanthanide magnetic momentsspin-only formulaorbital quenchingRussell–Saunders couplingLandé g factor

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