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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

Q4(a). MO diagram of NO; NO and NO⁺; NO as a radical

Question

Draw the molecular orbital (MO) diagram of NO molecule. The experimental bond dissociation energy of NO is 626 kJ mol⁻¹ while that of NO⁺ is 1047 kJ mol⁻¹ — rationalize it. NO can also act as a reactive radical — how?

Answer in brief

NO has one electron in an antibonding π* orbital (bond order 2.5); removing it gives NO⁺ with bond order 3, hence the stronger bond. The same odd electron makes NO a radical.

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Topic notes: molecular orbital theory: LCAO, MO diagrams of N₂, O₂, CO, NO, CN⁻ and HF, bond order and magnetism, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversmolecular orbital diagram of NObond ordernitrosonium ionbond dissociation energyparamagnetismfree radical

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