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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

Q5(c). Resonance structures of the nitrate ion

  • 5 marks
  • Answered
Question

Provide the resonance structures with formal charges for the nitrate ion.

Answer in brief

Three equivalent structures, each with one N=O and two N–O; formal charges N +1, double-bonded O 0, each single-bonded O -1. The hybrid has three equal bonds of order 4/3.

Key points

  • N forms three σ bonds and one π bond. The π bond can sit on any O, giving three equivalent structures; only electrons move, not nuclei.
  • FC=V-(unshared electrons)-(bonds) 3pt] N: 5-0-4=+1; O in N=O: 6-4-2=0; O in N–O: 6-6-1=-1.
  • Sum: +1+0+2(-1)=-1, the charge of the ion.
  • all three N–O bonds are equal (122 pm), each of bond order 4/3, and each O carries -2/3; planar D₃ₕ, like CO₃²-.

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. N forms three σ bonds and one π bond. The π bond can sit on any O, giving three equivalent structures; only electrons move, not nuclei.
  2. Formal charges: FC=V-(unshared electrons)-(bonds) 3pt] N: 5-0-4=+1; O in N=O: 6-4-2=0; O in N–O: 6-6-1=-1.
  3. Sum: +1+0+2(-1)=-1, the charge of the ion.
  4. The real ion is the resonance hybrid: all three N–O bonds are equal (122 pm), each of bond order 4/3, and each O carries -2/3; planar D₃ₕ, like CO₃²-.

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Model answer to Q5(c), 2026 Paper I: solution page 29
Solution page 29 of 55
Read the whole 2026 solution2026 question paper

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