# Explain how electrons are arranged in an atom, and describe how this arrangement changes when an atom has a greater number of electrons.

> Edexcel A-Level Chemistry (9CH0) — 1.1 Atomic structure · Explain · 5 marks

> Atoms contain sub-atomic particles arranged in a specific structure. The electrons in an atom occupy shells at different distances from the nucleus.

## Mark scheme (5 marks)

1. Electrons occupy shells (energy levels) around the nucleus
2. The first shell (closest to the nucleus) can hold a maximum of 2 electrons
3. The second and third shells can each hold a maximum of 8 electrons
4. Shells fill from the innermost shell outwards / electrons fill the lowest energy level first
5. As the number of electrons increases, a new (outer) shell begins to fill once the previous shell is full

## 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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