Describe how a covalent bond is formed between two non-metal atoms and explain why simple molecular substances such as hydrogen chloride have low boiling points.

AQA GCSE Chemistry (8462) — 4.2.1 Chemical bonds, ionic, covalent and metallic · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A covalent bond is formed by the sharing of a pair of electrons between two non‑metal atoms.
The shared electrons hold the two atoms together, creating a strong bond within the molecule.
The molecules are held together only by weak intermolecular forces.
Because little energy is needed to overcome these weak forces, the boiling point is low.

Examiner tips

  • Use the phrase "sharing of a pair of electrons" to show understanding of covalent bonding.
  • Explain that the weak forces are intermolecular, not intramolecular, to justify the low boiling point.
  • Keep the answer concise – 4 points can be earned with two short sentences.
  • Use UK spelling (e.g., "bonding", "forces").

Common mistakes

  • Confusing covalent bonds with ionic bonds – students may mention charge transfer instead of electron sharing.
  • Failing to specify that the weak forces are intermolecular, not intramolecular.
  • Writing a long paragraph instead of two clear points, losing marks for lack of structure.

Mark scheme (4 marks)

  1. A covalent bond is formed by the sharing of a pair of electrons (between two non-metal atoms).
  2. The shared electrons hold the two atoms together (strong bond within the molecule).
  3. There are only weak intermolecular forces between the molecules.
  4. Little energy is needed to overcome these weak intermolecular forces, so the boiling point is low.

Key terms in this question

covalent bond · simple molecular substance

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