Fluorine, chlorine, bromine and iodine are halogens in Group 7. A student adds a small amount of chlorine water to a solution of potassium iodide. Explain what the student would observe and why this reaction occurs. In your answer, refer to the electronic structures of the halogens involved.

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

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

Halogens react with solutions of other halide salts. The reactivity of halogens decreases down Group 7. Chlorine is more reactive than iodine.

Model answer (5 marks)

1. The solution turns brown or orange‑brown as iodine is released.
2. Chlorine displaces iodine from potassium iodide in a displacement reaction.
3. Chlorine is more reactive than iodine.
4. Chlorine has only two electron shells, whereas iodine has five; the outer shell of chlorine is closer to the nucleus.
5. Because the outer electrons are closer to the nucleus, chlorine can gain an electron more easily than iodine, giving it a stronger attraction for the incoming electron and making it more reactive.

Examiner tips

  • Use the word "displacement" to show you understand the reaction type.
  • Mention the colour change to demonstrate observation.
  • Explain the electronic shell difference to justify reactivity.
  • Show the link between shell distance and electron affinity.

Common mistakes

  • Saying the solution turns blue instead of brown/orange‑brown.
  • Confusing chlorine with fluorine in the electronic structure.
  • Omitting the displacement reaction mechanism.

Mark scheme (5 marks)

  1. The solution turns brown/orange-brown (or iodine is produced)
  2. Chlorine displaces iodine from potassium iodide (displacement reaction)
  3. Chlorine is more reactive than iodine
  4. Chlorine atoms have fewer electron shells than iodine atoms (chlorine has 2 shells / iodine has 5 shells), so the outer shell is closer to the nucleus
  5. Chlorine can gain an electron more easily than iodine (stronger attraction between nucleus and incoming electron), so chlorine is more reactive

Key terms in this question

halogen

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