# Explain why the bond angle in a water molecule (H₂O) is approximately 104.5°, which is less than the ideal tetrahedral angle of 109.5°.

> IB DP Chemistry Standard Level (2023 syllabus) — S2.2 The covalent model · Explain · 4 marks

## Mark scheme (4 marks)

1. The central oxygen atom has four electron domains (two bonding pairs and two lone pairs), giving a tetrahedral electron geometry.
2. Lone pairs occupy greater angular space / repel more strongly than bonding pairs.
3. The two lone pairs on oxygen repel the two O–H bonding pairs more strongly than bonding pairs repel each other.
4. The H–O–H bond angle is compressed below 109.5° to approximately 104.5° as a result of this increased lone pair repulsion.

## Key terms

- [bond angle](https://www.gradenine.co.uk/glossary/bond-angle)
- [tetrahedral](https://www.gradenine.co.uk/glossary/tetrahedral)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-bond-angle-in-f6f63811) · Published by Druglandscape Ltd.