Chlorine, bromine, and iodine are all elements in Group 7 of the periodic table, known as the halogens. Explain why these three elements are placed in the same group, and describe the trend in reactivity going down Group 7, giving a reason for this trend.

OCR A-Level Chemistry B: Salters (H433) — 3.1 The periodic table and periodicity · Explain · 5 marks · View as Markdown

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

The halogens are a group of non-metals that form salts when they react with metals. Chlorine is a pale yellow-green gas at room temperature, bromine is a brown liquid, and iodine is a grey-black solid.

Model answer (5 marks)

All three elements have the same number of valence electrons (seven) because they are in the same group of the periodic table. The same outer‑electron configuration gives them similar chemical properties, such as forming salts with metals.

Reactivity decreases down Group 7. Chlorine is the most reactive, iodine the least. This is because the atoms have more electron shells as you go down the group, so the outer electrons are further from the nucleus. The increased distance reduces the nucleus’s attraction for an extra electron, making it harder for the halogen to gain an electron and complete its outer shell.

Examiner tips

  • State the common valence‑electron count first, then link to similar properties. Explain the trend by mentioning shell number, electron distance and nuclear attraction. Use the exact terms ‘valence electrons’, ‘reactivity’, ‘electron shells’ and ‘nucleus’ as in the mark scheme.

Common mistakes

  • Confusing the trend (writing reactivity increases down the group). Failing to mention the number of valence electrons. Using vague phrases like ‘more electronegative’ instead of the specific reason given in the mark scheme.

Mark scheme (5 marks)

  1. Elements in the same group have the same number of electrons in their outer shell / same number of outer electrons
  2. Elements in the same group have similar chemical properties (because of the same number of outer electrons)
  3. Reactivity decreases going down Group 7 (chlorine is most reactive, iodine is least reactive)
  4. Going down the group, atoms have more electron shells / the outer electrons are further from the nucleus
  5. The nucleus has less attraction for / finds it harder to attract an additional electron, so it is harder to gain an electron to complete the outer shell

Key terms in this question

Group 7 · halogens · reactivity

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