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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 · 2026 · Paper I

Q8(c)(iv). Why Ce³⁺ and Yb³⁺ are colourless but absorb in the UV

  • 5 marks
  • Answered
Question

Why do Ce³⁺ and Yb³⁺ not absorb in the visible region, but show sharp absorption in the ultraviolet region?

Answer in brief

4f¹ and 4f¹³ have only one excited f level, in the infrared, so there is no f–f band in the visible: both are colourless. The intense UV band is a Laporte-allowed 4f → 5d transition (Ce³⁺) or a ligand-to-metal charge transfer (Yb³⁺).

Key points

  • The UV bands are intense (f→d and charge transfer), unlike the weak, sharp f–f lines.
  • One f electron (or hole) gives one term, ²F; spin–orbit coupling splits it into only two levels, J=52 and 72.
  • That single gap lies in the infrared. For Yb³⁺ it is near 1 µm (Yb lasers emit at 1.030 µm, about 9700 cm⁻¹): a sharp but weak line outside the visible (400–700 nm). Hence both ions are colourless.
  • 4f → 5d has Δ l=1, so it is Laporte allowed and strong. Yb³⁺ is easily reduced to Yb²⁺ (4f¹⁴), so ligand → metal charge transfer gives an intense band. Both lie in the UV.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. The UV bands are intense (f→d and charge transfer), unlike the weak, sharp f–f lines.
  2. One f electron (or hole) gives one term, ²F; spin–orbit coupling splits it into only two levels, J=52 and 72.
  3. That single gap lies in the infrared. For Yb³⁺ it is near 1 µm (Yb lasers emit at 1.030 µm, about 9700 cm⁻¹): a sharp but weak line outside the visible (400–700 nm). Hence both ions are colourless.
  4. Ce³⁺: 4f → 5d has Δ l=1, so it is Laporte allowed and strong. Yb³⁺ is easily reduced to Yb²⁺ (4f¹⁴), so ligand → metal charge transfer gives an intense band. Both lie in the UV.

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Model answer to Q8(c)(iv), 2026 Paper I: solution page 55
Solution page 55 of 55
Read the whole 2026 solution2026 question paper

All parts of Question 8