Explain why iodine (I₂) is a solid at room temperature but does not conduct electricity in any state.

Cambridge International IGCSE Chemistry (0620) — 2.5 Simple molecules and covalent bonds · Explain · 4 marks · View as Markdown

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

Iodine is a shiny, grey-purple solid at room temperature. It is a simple molecular substance made up of I₂ molecules.

Model answer (4 marks)

Iodine molecules are held together by weak London dispersion forces.
These forces are strong enough to keep the I₂ molecules in a fixed arrangement, so iodine is a solid at room temperature.
In iodine there are no free or mobile electrons (or ions).
Electrical conductivity requires charged particles that can move, so iodine cannot conduct electricity in any state.

Examiner tips

  • Use the term ‘London dispersion forces’ to show knowledge of intermolecular forces.
  • Explain that conductivity needs mobile charges – mention electrons or ions.
  • Show the logical link: weak forces → solid state, no mobile charges → no conductivity.
  • Keep the answer concise and use the exact wording from the mark scheme.

Common mistakes

  • Confusing covalent bonds with ionic bonds – students may say iodine conducts because it is covalent.
  • Failing to mention that no free electrons or ions are present.
  • Using vague phrases like ‘weak forces’ without specifying London dispersion forces.

Mark scheme (4 marks)

  1. Iodine molecules are held together by weak intermolecular forces
  2. These weak intermolecular forces are strong enough to keep the molecules in a fixed arrangement / together as a solid at room temperature
  3. There are no free / mobile electrons (or ions) in iodine
  4. Electrical conductivity requires charged particles that can move / flow, so iodine cannot conduct in any state

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