Water (H₂O) is a covalently bonded molecule. Explain why a water molecule has a bent (non-linear) shape rather than a straight (linear) shape, and state the bond angle found in a water molecule.

Edexcel A-Level Chemistry (9CH0) — 2.2 Covalent bonding and shapes · Explain · 5 marks · View as Markdown

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

Water is essential for life. Each water molecule contains two hydrogen atoms covalently bonded to one oxygen atom. Oxygen has six electrons in its outer shell.

Model answer (5 marks)

A covalent bond is formed by a shared pair of electrons between oxygen and each hydrogen.
Oxygen forms two covalent bonds, leaving two lone pairs of electrons on the oxygen atom.
Lone pairs repel more strongly than bonding pairs.
The lone pairs push the bonding pairs closer together, bending the molecule away from a linear arrangement.
The H–O–H bond angle is approximately 104.5°.

Examiner tips

  • Use the term ‘lone pair’ and ‘bonding pair’ explicitly. Explain the repulsion hierarchy (lone pair > bonding pair). State the exact bond angle (104.5°).

Common mistakes

  • Forgetting to mention the two lone pairs on oxygen. Using ‘bent’ without explaining the repulsion. Giving an incorrect bond angle such as 109.5° or 90°.

Mark scheme (5 marks)

  1. A covalent bond is formed by a shared pair of electrons (between oxygen and each hydrogen)
  2. Oxygen forms two covalent bonds, leaving two lone pairs of electrons on the oxygen atom
  3. Lone pairs repel more strongly than bonding pairs
  4. The lone pairs push the bonding pairs closer together, bending the molecule away from a linear arrangement
  5. The H–O–H bond angle is approximately 104.5°

Key terms in this question

bond angle

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