Explain why the bond angle in water (H₂O) is less than the bond angle in ammonia (NH₃), and why both are less than the tetrahedral angle of 109.5°.

IB DP Chemistry Higher Level (2023 syllabus) — S2.2 The covalent model · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Both water and ammonia have a tetrahedral electron‑domain geometry, so the ideal bond angle is 109.5°. In each molecule the lone pairs occupy more space than bonding pairs because they are held closer to the nucleus and are not shared between two atoms. Ammonia has one lone pair and three bonding pairs; the lone pair compresses the H–N–H angles to about 107°, below 109.5°. Water has two lone pairs and only two bonding pairs; the two lone pairs exert even greater repulsion, compressing the H–O–H angle to about 104.5°, which is smaller than in ammonia.

Examiner tips

  • State the tetrahedral reference angle first
  • Explain lone‑pair vs bonding‑pair repulsion
  • Give the approximate angles for NH3 and H2O

Common mistakes

  • Confusing the number of lone pairs with the angle magnitude
  • Using the wrong reference angle (e.g. 90° instead of 109.5°)
  • Failing to mention that lone pairs are closer to the nucleus

Mark scheme (4 marks)

  1. Both water and ammonia have a tetrahedral electron domain geometry (four electron domains around the central atom), so the ideal/reference angle is 109.5°.
  2. Lone pairs occupy more space / exert greater repulsion than bonding pairs because lone pair electrons are held closer to the central atom and are not constrained between two nuclei.
  3. Ammonia has one lone pair and three bonding pairs; the one lone pair compresses the three H–N–H bond angles below 109.5° to approximately 107°.
  4. Water has two lone pairs and only two bonding pairs; the greater lone pair repulsion (two lone pairs) compresses the H–O–H bond angle further to approximately 104.5°, which is less than in ammonia.

Key terms in this question

bond angle · tetrahedral

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