Explain why iodine (I₂) does not conduct electricity in any state and has a low melting point.

Edexcel GCSE Chemistry (1CH0) — 1.6 Covalent bonding · Explain · 4 marks · View as Markdown

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

Iodine is a dark grey solid at room temperature. It consists of diatomic molecules held together by weak intermolecular forces. Iodine does not conduct electricity whether solid or liquid.

Model answer (4 marks)

Iodine molecules are held together by weak London dispersion forces, which are intermolecular forces. Because these forces are weak, only a small amount of energy is required to break them, giving iodine a low melting point.

Iodine contains no free or delocalised electrons and no ions are present in any state. Consequently there are no mobile charge carriers.

Without mobile charge carriers, iodine cannot conduct electricity in the solid or liquid state.

Examiner tips

  • Use the word ‘intermolecular’ to show understanding of London forces. Show the link between weak forces and low melting point. State that iodine has no free electrons or ions. Explain that lack of mobile charge carriers means no conductivity.

Common mistakes

  • Confusing covalent bonds with ionic bonds. Mentioning ‘molecular weight’ instead of ‘intermolecular forces’. Forgetting to explain the absence of mobile charge carriers.

Mark scheme (4 marks)

  1. Iodine molecules are held together by weak intermolecular forces (between molecules)
  2. Little energy is needed to overcome these weak intermolecular forces, so iodine has a low melting point
  3. Iodine has no free/delocalised electrons (or ions) in any state
  4. Therefore iodine cannot conduct electricity because there are no mobile charge carriers

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