# Sodium (Na) and potassium (K) are both elements in Group 1 of the periodic table. Chlorine (Cl) and bromine (Br) are both elements in Group 7. Explain why elements in the same group of the periodic table have similar chemical properties, and describe one trend in reactivity as you move down Group 1 and one trend in reactivity as you move down Group 7.

> OCR A-Level Chemistry A (H432) — 3.1 The periodic table and periodicity · Explain · 5 marks

> The periodic table arranges elements in order of increasing atomic number. Elements in the same group have the same number of electrons in their outermost shell. Group 1 metals react vigorously with water, and Group 7 non-metals react with metals to form salts.

## Mark scheme (5 marks)

1. Elements in the same group have the same number of electrons in their outermost (outer) shell
2. Because they have the same number of outer electrons, elements in the same group react in the same / similar way (giving them similar chemical properties)
3. Reactivity increases as you go down Group 1
4. Reactivity decreases as you go down Group 7
5. A correct reason for either Group 1 or Group 7 reactivity trend linked to the outer shell / ease of losing or gaining an electron (e.g. outer shell is further from the nucleus so electron is lost more easily in Group 1, OR outer shell is further from the nucleus so gaining an electron is harder in Group 7)

## Key terms

- [group](https://www.gradenine.co.uk/glossary/group)
- [reactivity](https://www.gradenine.co.uk/glossary/reactivity)
- [periodic table](https://www.gradenine.co.uk/glossary/periodic-table)

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](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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