Chlorine, bromine and iodine are all halogens in Group 7. Explain the trend in reactivity down Group 7 and describe what is observed when chlorine water is added to a solution of potassium iodide.

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).

The halogens become less reactive going down Group 7. Chlorine is more reactive than iodine.

Model answer (5 marks)

Reactivity decreases down Group 7 because the atoms become larger and have more electron shells, so the outermost electrons are further from the nucleus and are less strongly attracted. Consequently it is harder for the halogen to gain an electron.

When chlorine water is added to a potassium iodide solution, chlorine displaces iodine from the complex ion. Iodine is released and the solution turns brown/orange‑brown.

Examiner tips

  • State the size/electron‑shell argument for decreasing reactivity. Explain the displacement reaction and the colour change. Use the term ‘displaces’ and mention iodine’s colour change.
  • common_mistakes
  • :
  • Confusing the trend (e.g. saying reactivity increases down the group). Failing to mention the colour change or the displacement mechanism. Using vague terms like ‘more reactive’ without explanation.

Mark scheme (5 marks)

  1. Reactivity decreases going down Group 7
  2. Because atoms get larger / have more electron shells going down the group, so the outer shell is further from the nucleus
  3. So it is harder to gain an electron / attract an electron into the outer shell
  4. Chlorine displaces iodine from potassium iodide solution (because chlorine is more reactive)
  5. The solution turns brown / orange-brown (as iodine is produced)

Key terms in this question

reactivity · Group 7 · halogens

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