Chlorine, bromine and iodine are all elements in Group 7 of the periodic table. Describe how the reactivity of the Group 7 elements changes as you go down the group, and explain why this trend occurs in terms of atomic structure.

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

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

Model answer (5 marks)

Reactivity decreases down Group 7.

As you move down the group the atoms have more electron shells.

The outermost shell is further from the nucleus, so the nuclear attraction for an incoming electron is weaker.

Group 7 elements react by gaining one electron to form a halide ion.

Because it is harder to attract an electron as you go down the group, each successive element is less reactive.

Examiner tips

  • Use the word "decreases" to show the trend. Mention the increase in electron shells. Explain the weaker nuclear attraction. State the halide ion formation. Link weaker attraction to lower reactivity.

Common mistakes

  • Saying reactivity increases instead of decreases. Forgetting to mention the halide ion. Using vague terms like "less" without explaining why.

Mark scheme (5 marks)

  1. Reactivity decreases going down Group 7
  2. Going down the group, the atoms have more electron shells
  3. The outer shell is further from the nucleus, so the nucleus has less attraction for an incoming electron
  4. Group 7 elements react by gaining one electron to form a negative ion (halide ion)
  5. Because it is harder to attract an electron as you go down the group, each successive element is less reactive

Key terms in this question

Group 7 · reactivity

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