# Fluorine, chlorine, bromine and iodine are halogens in Group 7. A student adds a small amount of chlorine water to a solution of potassium iodide. Explain what the student would observe and why this reaction occurs. In your answer, refer to the electronic structures of the halogens involved.

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

> Halogens react with solutions of other halide salts. The reactivity of halogens decreases down Group 7. Chlorine is more reactive than iodine.

## Mark scheme (5 marks)

1. The solution turns brown/orange-brown (or iodine is produced)
2. Chlorine displaces iodine from potassium iodide (displacement reaction)
3. Chlorine is more reactive than iodine
4. Chlorine atoms have fewer electron shells than iodine atoms (chlorine has 2 shells / iodine has 5 shells), so the outer shell is closer to the nucleus
5. Chlorine can gain an electron more easily than iodine (stronger attraction between nucleus and incoming electron), so chlorine is more reactive

## Key terms

- [halogen](https://www.gradenine.co.uk/glossary/halogen)

## 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/fluorine-chlorine-bromine-and-iodine-are-b51baffe) · Published by Druglandscape Ltd.