Explain why the bond angle in sulfur dioxide (SO₂) is approximately 119°, which is less than the bond angle of 120° found in a molecule with an ideal trigonal planar arrangement.

IB DP Chemistry Standard 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).

Sulfur dioxide (SO₂) is a bent molecule with a bond angle of approximately 119°.

Model answer (4 marks)

Sulfur dioxide has a central S atom with three electron domains – two S–O bonds and one lone pair. According to VSEPR, the electron‑domain geometry is trigonal planar, but the lone pair exerts a stronger repulsion than a bonding pair. This extra repulsion pushes the two bonding pairs closer together, reducing the H–S–H angle from the ideal 120° to about 119°. Consequently the molecular shape is bent (V‑shaped) rather than trigonal planar.

Examiner tips

  • State the electron‑domain count and geometry first. Explain the lone‑pair repulsion effect on bond angles. Show the resulting bent shape. Use the exact wording from the mark scheme.

Common mistakes

  • Confusing the bond angle with the H–S–H angle. Saying the lone pair is on the oxygen. Giving the wrong electron‑domain geometry (tetrahedral).

Mark scheme (4 marks)

  1. The central sulfur atom has three electron domains (two bonding domains and one lone pair), giving a trigonal planar electron domain geometry.
  2. The lone pair on sulfur repels the bonding pairs more strongly than bonding pair–bonding pair repulsion.
  3. This greater repulsion from the lone pair compresses / pushes the two bonding pairs closer together.
  4. The molecular shape of SO₂ is bent / V-shaped (not trigonal planar) because one of the three electron domains is a lone pair and does not contribute to the molecular geometry.

Key terms in this question

bond angle · trigonal planar

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