# Chlorine, bromine and iodine are all halogens in Group 7. Explain the trend in reactivity down Group 7 and describe what is observed when chlorine water is added to a solution of potassium iodide.

> Edexcel A-Level Chemistry (9CH0) — 4.2 Group 7 halogens · Explain · 5 marks

> The halogens become less reactive going down Group 7. Chlorine is more reactive than iodine.

## Mark scheme (5 marks)

1. Reactivity decreases going down Group 7
2. Because atoms get larger / have more electron shells going down the group, so the outer shell is further from the nucleus
3. So it is harder to gain an electron / attract an electron into the outer shell
4. Chlorine displaces iodine from potassium iodide solution (because chlorine is more reactive)
5. The solution turns brown / orange-brown (as iodine is produced)

## Key terms

- [reactivity](https://www.gradenine.co.uk/glossary/reactivity)
- [Group 7](https://www.gradenine.co.uk/glossary/group-7)
- [halogens](https://www.gradenine.co.uk/glossary/halogens)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/chlorine-bromine-and-iodine-are-all-e08381b2) · Published by Druglandscape Ltd.