# Chlorine, bromine, and iodine are all elements in Group 7 of the periodic table, known as the halogens. Explain why these three elements are placed in the same group, and describe the trend in reactivity going down Group 7, giving a reason for this trend.

> OCR A-Level Chemistry B: Salters (H433) — 3.1 The periodic table and periodicity · Explain · 5 marks

> The halogens are a group of non-metals that form salts when they react with metals. Chlorine is a pale yellow-green gas at room temperature, bromine is a brown liquid, and iodine is a grey-black solid.

## Mark scheme (5 marks)

1. Elements in the same group have the same number of electrons in their outer shell / same number of outer electrons
2. Elements in the same group have similar chemical properties (because of the same number of outer electrons)
3. Reactivity decreases going down Group 7 (chlorine is most reactive, iodine is least reactive)
4. Going down the group, atoms have more electron shells / the outer electrons are further from the nucleus
5. The nucleus has less attraction for / finds it harder to attract an additional electron, so it is harder to gain an electron to complete the outer shell

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](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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