Iodine is a halogen in Group 7 of the periodic table. When iodine reacts with potassium metal, potassium iodide is formed. Explain why iodine is a less reactive halogen than chlorine, and state the type of ion formed when iodine reacts with potassium.

Pearson Edexcel International GCSE Chemistry (4CH1) — 2.2 Group 7 (halogens) · Explain · 4 marks · View as Markdown

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

Halogens react with metals to form ionic compounds called metal halides. The reactivity of halogens decreases down Group 7.

Model answer (4 marks)

Iodine atoms are larger than chlorine atoms, having more electron shells. The outer shell electron is further from the nucleus, so the nucleus attracts an incoming electron less strongly. Therefore iodine gains an electron less readily than chlorine, making it less reactive. Iodine forms an iodide ion, I⁻, with a 1− charge.

Examiner tips

  • Use the word ‘less’ to compare reactivity; mention size/electron shells. Include the charge on the ion. Keep the explanation concise and use correct terminology.

Common mistakes

  • Saying iodine is more reactive than chlorine. "Iodine is a larger atom" without linking to reactivity. Forgetting to state the 1− charge on the iodide ion.

Mark scheme (4 marks)

  1. Iodine atoms are larger than chlorine atoms / iodine has more electron shells than chlorine
  2. The outer shell / incoming electron is further from the nucleus in iodine, so the nucleus attracts an incoming electron less strongly
  3. Therefore iodine gains an electron less easily / less readily than chlorine, making it less reactive
  4. Iodine forms an iodide ion / I⁻ ion with a 1− charge

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