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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 · 2025 · Paper I

Q8(d). Magnetic moments of Pr³⁺ and Tb³⁺

Question

Write electronic configuration (outer) of the following lanthanide ions and calculate the magnetic moment in BM from LS coupling. (i) Pr³⁺ (g = 4/5) (ii) Tb³⁺ (g = 3/2)

Answer in brief

Pr³⁺: [Xe]4f², μ = 3.58 BM; Tb³⁺: [Xe]4f⁸, μ = 9.72 BM.

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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 momentLS couplingLandé g factorHund's rulesterm symbolPr3+ Tb3+

All parts of Question 8