Fluorine, chlorine, bromine and iodine are all elements in Group 7 of the periodic table, known as the halogens. Explain how the reactivity of the halogens changes as you move down Group 7, linking this trend to changes in atomic structure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The halogens are a group of non-metals that react with metals and hydrogen to form salts. Their physical and chemical properties show clear periodic trends.
Model answer (5 marks)
Reactivity decreases down Group 7.
1. All halogens need one electron to complete their valence shell.
2. Moving down the group the number of electron shells increases, so the atoms become larger.
3. The outermost shell is further from the nucleus, reducing the nuclear attraction for an incoming electron.
4. Weaker attraction makes it harder to gain an electron, so the element is less reactive.
1. All halogens need one electron to complete their valence shell.
2. Moving down the group the number of electron shells increases, so the atoms become larger.
3. The outermost shell is further from the nucleus, reducing the nuclear attraction for an incoming electron.
4. Weaker attraction makes it harder to gain an electron, so the element is less reactive.
Examiner tips
- State the trend first, then give the atomic‑structure reason; use terms like ‘electron shells’, ‘nuclear attraction’, ‘electron‑gain reactivity’.
- Show the logical sequence: size ↑ → distance ↑ → attraction ↓ → reactivity ↓.
Common mistakes
- Confusing reactivity trend with electronegativity trend; halogens become less reactive, not more.
- Failing to mention the need for one electron to complete the octet or to describe the role of electron shells.
Mark scheme (5 marks)
- Reactivity decreases as you go down Group 7
- Each element in Group 7 needs to gain one electron to achieve a full outer shell
- As you go down the group, the atoms have more electron shells / are larger
- The outer shell is further from the nucleus as you go down the group, so the attraction between the nucleus and an incoming electron is weaker
- Therefore, it becomes harder to gain an electron, so the element is less reactive
Key terms in this question
Group 7 · halogens · reactivity
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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