Explain why elements in Group 7 of the periodic table become less reactive as you go down the group.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Group 7 elements are known as the halogens. Fluorine is the most reactive halogen, and reactivity decreases going down the group from fluorine to astatine.
Model answer (5 marks)
Halogens gain one electron to achieve a full outer shell.
1. As you move down Group 7 the atoms become larger, having more electron shells.
2. The outermost electron is therefore further from the nucleus.
3. Inner‑shell electrons shield the outer electron from the nuclear charge.
4. Shielding increases down the group, reducing the effective nuclear charge felt by the outer electron.
5. Consequently the nucleus attracts the incoming electron less strongly, making it harder to gain an electron and the element less reactive.
1. As you move down Group 7 the atoms become larger, having more electron shells.
2. The outermost electron is therefore further from the nucleus.
3. Inner‑shell electrons shield the outer electron from the nuclear charge.
4. Shielding increases down the group, reducing the effective nuclear charge felt by the outer electron.
5. Consequently the nucleus attracts the incoming electron less strongly, making it harder to gain an electron and the element less reactive.
Examiner tips
- Use the term ‘gain one electron’ and ‘full outer shell’. Explain size, distance, shielding and effective nuclear charge in order. Show the logical chain from size to reactivity.
Common mistakes
- Saying reactivity increases down the group. Forgetting to mention shielding or effective nuclear charge. Using vague terms like ‘more electrons’ without linking to distance or shielding.
Mark scheme (5 marks)
- Halogens react by gaining one electron (to complete their outer shell / to form a negative ion / to achieve a full outer shell)
- As you go down Group 7, the atoms get larger / have more electron shells
- The outer shell / incoming electron is further from the (positive) nucleus
- There is greater shielding / shielding by inner electrons increases going down the group
- So the nucleus attracts the incoming electron less strongly / it is harder to gain an electron, making the element less reactive
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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