Fluorine, chlorine and bromine are all elements in Group 7 of the periodic table. When chlorine gas is bubbled through a solution of potassium bromide, a reaction takes place. Explain why this reaction occurs and predict whether fluorine would react in a similar way with potassium bromide solution, justifying your answer. Include reference to electron shells in your answer.

WJEC A-Level Chemistry (Wales) — 2.2 Group 1, 2 and 7 · Explain · 5 marks · View as Markdown

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

Group 7 elements are also known as the halogens. Their reactivity changes as you move down the group.

Model answer (5 marks)

1. Chlorine is higher in Group 7 than bromine, so it is more reactive.
2. A more reactive halogen can displace a less reactive halogen from its salt.
3. In the reaction, Cl₂ accepts an electron from Br⁻, forming Cl⁻ and Br₂.
4. Down the group the outer electron shell is further from the nucleus, making electron gain harder and reactivity lower.
5. Fluorine is above chlorine in Group 7, has fewer electron shells and is the most reactive halogen, so it would also displace bromide from potassium bromide solution.

Examiner tips

  • Use the term "displacement reaction" and mention electron transfer. Show the relative positions in the group. Explain electron shell distance. State fluorine’s higher reactivity explicitly.

Common mistakes

  • Confusing reactivity with electronegativity. Saying bromine reacts with chlorine instead of chlorine displacing bromine. Ignoring the role of electron shells in explaining reactivity.

Mark scheme (5 marks)

  1. Chlorine is more reactive than bromine (accept: chlorine is higher/above bromine in Group 7)
  2. A more reactive halogen displaces a less reactive halogen from its salt solution (accept: chlorine displaces bromide ions from the solution)
  3. As you go down Group 7, atoms have more electron shells / the outer electron shell is further from the nucleus
  4. It is harder to gain an electron as you go down the group, so reactivity decreases going down Group 7 (accept: chlorine gains an electron more easily than bromine)
  5. Fluorine would react with potassium bromide solution because fluorine is more reactive than chlorine / fluorine is above chlorine in Group 7 / fluorine has fewer electron shells so gains an electron most easily

Key terms in this question

Group 7 · electron shells

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