VKC ScienceChemistry for written exams Telegram

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 · 2016 · Paper I

Q1(a)(i). Bond angles of CH₄, NH₃ and H₂O

  • 5 marks
  • Answered
Question

The bond angles in CH₄, NH₃ and H₂O molecules are 109.5°, 107° and 104.5° respectively. Explain.

Answer in brief

All three central atoms carry four electron pairs in a tetrahedral arrangement. Lone pairs repel more strongly than bond pairs, so each lone pair added closes the H–X–H angle: 109.5° (no lone pair), 107° (one), 104.5° (two).

Key points

  • C, N and O each have four electron pairs in the valence shell (sp³). Four pairs keep apart best at the corners of a tetrahedron.
  • four identical bond pairs, so a perfect tetrahedron: 109.5° (the tetrahedral angle, cos⁻¹(-13)).
  • three bond pairs and one lone pair. The lone pair is held by one nucleus only and spreads closer to N; its larger repulsion pushes the three N–H bonds together: 107°
  • two bond pairs and two lone pairs. lp–lp repulsion is the largest of all, so the two O–H bonds are squeezed further: 104.5°

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. C, N and O each have four electron pairs in the valence shell (sp³). Four pairs keep apart best at the corners of a tetrahedron.
  2. CH₄: four identical bond pairs, so a perfect tetrahedron: 109.5° (the tetrahedral angle, cos⁻¹(-13)).
  3. NH₃: three bond pairs and one lone pair. The lone pair is held by one nucleus only and spreads closer to N; its larger repulsion pushes the three N–H bonds together: 107°
  4. H₂O: two bond pairs and two lone pairs. lp–lp repulsion is the largest of all, so the two O–H bonds are squeezed further: 104.5°
  5. The same trend in hybrid language: the lone pairs take more s character, leaving more p character in the bonding orbitals, and p orbitals meet at 90°. The shape seen is tetrahedral, pyramidal and bent.

Practising this answer? Write it in a Mains test and get it checked by VKC. Civil Services Mains test series · Telegram

Model answer to Q1(a)(i), 2016 Paper I: solution page 2
Solution page 2 of 46
Read the whole 2016 solution2016 question paper

All parts of Question 1