Chlorine, bromine and iodine are halogens in Group 7. A student investigates the physical properties and bonding of these elements. Describe how the physical properties of the halogens change going down Group 7 from chlorine to iodine, and explain why these changes occur in terms of atomic structure and intermolecular forces.

Edexcel A-Level Chemistry (9CH0) — 4.2 Group 7 halogens · Describe · 5 marks · View as Markdown

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

Chlorine is a pale yellow-green gas at room temperature. Bromine is a dark orange-brown liquid at room temperature. Iodine is a shiny grey-black solid at room temperature.

Model answer (5 marks)

The boiling point (and melting point) rises from chlorine to iodine. The atoms become larger, having more electron shells, so the electron cloud is more polarizable. Larger, more polarizable atoms give stronger London dispersion forces between molecules. Stronger dispersion forces mean more energy is required to separate the molecules, so the boiling point is higher. The colour also becomes darker – chlorine is pale yellow‑green, bromine orange‑brown, iodine grey‑black.

Examiner tips

  • Use the word ‘rises’ for boiling point, not ‘increases’
  • Mention electron shells and polarizability
  • Explain that stronger London forces raise boiling point
  • State the colour change in order

Common mistakes

  • Confusing boiling point with melting point
  • Saying ‘more energy is needed to break bonds’ instead of ‘intermolecular forces’
  • Omitting the link between larger atoms and stronger London forces

Mark scheme (5 marks)

  1. The boiling point (or melting point) increases going down Group 7 from chlorine to iodine
  2. The atoms become larger / have more electron shells going down Group 7
  3. Larger atoms / molecules have stronger London dispersion forces (van der Waals forces / intermolecular forces) between molecules
  4. More energy is needed to overcome the stronger intermolecular forces, so the boiling point is higher
  5. The colour of the halogens becomes darker / deeper going down Group 7 (chlorine is pale yellow-green, bromine is orange-brown, iodine is grey-black)

Key terms in this question

halogens · Group 7 · intermolecular forces

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