Describe how the reactivity of the halogens changes going down Group VII, and explain this trend in terms of electronic structure.

Cambridge International IGCSE Chemistry (0620) — 8.3 Group VII properties · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Reactivity decreases down Group VII.

Halogens gain one electron to achieve a noble‑gas configuration.

As you move down the group the atoms have more electron shells; the valence shell is further from the nucleus.

The attraction between the nucleus and an incoming electron is therefore weaker, making it harder for the atom to gain an electron and so the reactivity falls.

Examiner tips

Use the word ‘decreases’ to show the trend; mention the outer shell moving outward.
Explain that halogens gain one electron to complete the octet.
Highlight the weaker nuclear attraction due to increased distance and shielding.
Keep the answer short and use the exact terminology from the mark scheme.

Common mistakes

Saying reactivity increases or that halogens lose electrons.
Forgetting to mention the octet completion or the outer shell distance.
Using vague terms like ‘less reactive’ without explaining the electronic reason.

Mark scheme (4 marks)

Reactivity decreases going down Group VII
Halogens react by gaining one electron (to complete their outer shell)
Going down the group, atoms have more electron shells / the outer shell is further from the nucleus
The attraction between the nucleus and an incoming electron is weaker, so it is harder to gain an electron

Key terms in this question

Group VII · reactivity