Explain why the bond angle in sulfur dioxide (SO₂) differs from the bond angle in carbon dioxide (CO₂), with reference to electron domains and molecular geometry.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
CO₂ has two bonding domains (double bonds) and no lone pairs on the central carbon, giving a linear geometry with a bond angle of 180°.
SO₂ has two bonding domains and one lone pair on the central sulfur, giving a bent/V‑shaped geometry.
The lone pair in SO₂ repels the bonding pairs more strongly than bonding pair–bonding pair repulsion, compressing the O–S–O bond angle below 120°.
Therefore the bond angle in SO₂ (~119°) is significantly less than in CO₂ (180°) because SO₂ is bent while CO₂ is linear.
SO₂ has two bonding domains and one lone pair on the central sulfur, giving a bent/V‑shaped geometry.
The lone pair in SO₂ repels the bonding pairs more strongly than bonding pair–bonding pair repulsion, compressing the O–S–O bond angle below 120°.
Therefore the bond angle in SO₂ (~119°) is significantly less than in CO₂ (180°) because SO₂ is bent while CO₂ is linear.
Examiner tips
- Use the term ‘electron domains’ to show you understand VSEPR; mention ‘two bonding domains + one lone pair’ for SO₂. State the bond angles (180° vs ~119°) to show the quantitative difference. Explain the repulsion hierarchy: lone pair > bonding pair. Keep the answer concise – 4 points can be covered in 4 short sentences.
Common mistakes
- Mixing up the number of electron domains – forgetting the lone pair on S. Using ‘bonding pairs’ instead of ‘electron domains’. Giving the wrong bond angle for SO₂ (e.g., 120°) or not stating the compression effect.
Mark scheme (4 marks)
- CO₂ has two bonding domains (double bonds) and no lone pairs on the central carbon, giving a linear geometry with a bond angle of 180°.
- SO₂ has two bonding domains and one lone pair on the central sulfur, giving a bent/V-shaped geometry.
- The lone pair in SO₂ repels the bonding pairs more strongly than bonding pair–bonding pair repulsion, compressing the O–S–O bond angle below 120°.
- Therefore the bond angle in SO₂ (~119°) is significantly less than in CO₂ (180°) because SO₂ is bent while CO₂ is linear.
Key terms in this question
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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