Astatine is a halogen found below iodine in Group 7. A student carries out displacement reactions using solutions of potassium fluoride, potassium chloride, potassium bromide and potassium iodide. The student adds astatine solution to each. Predict and explain the results the student would observe, and state which solution(s) would show a positive result.

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).

Astatine is a very rare halogen at the bottom of Group 7. Like the other halogens, it can take part in displacement reactions with halide salts.

Model answer (5 marks)

Astatine is the least reactive halogen in Group 7. A more reactive halogen can displace a less reactive one from its salt, but astatine is less reactive than F⁻, Cl⁻, Br⁻ and I⁻. Therefore it will not displace any of these halides. No colour change or precipitation will be observed in any of the four solutions.

The positive result would be none – all four solutions will show no reaction.

Examiner tips

  • State that astatine is least reactive in Group 7; link to reactivity trend. Mention displacement requires added halogen to be more reactive. Note that no reaction occurs in any solution. Keep answer concise and use correct terminology.

Common mistakes

  • Claiming astatine can displace iodine or bromine; confusing reactivity order. Writing that a colour change occurs in one solution. Using vague terms like "reactivity" without explaining the trend.

Mark scheme (5 marks)

  1. Astatine is the least reactive halogen (in Group 7) / less reactive than fluorine, chlorine, bromine and iodine
  2. A more reactive halogen displaces a less reactive halogen from its salt / displacement only occurs when the added halogen is more reactive than the halide in solution
  3. Astatine will NOT displace fluoride, chloride, bromide or iodide ions / none of the four solutions will show a positive result / no colour change is observed in any solution
  4. Reactivity of halogens decreases down Group 7 because the atoms get larger / the outer shell gets further from the nucleus (as you go down the group)
  5. The ability to attract / gain an electron to form a halide ion decreases down Group 7, making astatine the least able to gain an electron and therefore least reactive

Key terms in this question

displacement · halogen · Group 7

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