# Explain how the relative masses and relative charges of the subatomic particles in an atom account for the overall mass and charge of the atom.

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

> An atom of sodium has 11 protons, 12 neutrons and 11 electrons.

## Mark scheme (5 marks)

1. Protons and neutrons are both assigned a relative mass of 1 (each), so they contribute to the total mass of the atom
2. Electrons have a relative mass of approximately 0 (or negligible/very small), so they do not contribute significantly to the mass of the atom
3. The mass number (or relative atomic mass) equals the number of protons plus the number of neutrons, giving sodium a mass of 23
4. Protons have a relative charge of +1 and electrons have a relative charge of −1; neutrons have no charge (relative charge of 0)
5. The number of protons equals the number of electrons, so the positive and negative charges cancel out, meaning the atom has no overall charge

## Key terms

- [relative mass](https://www.gradenine.co.uk/glossary/relative-mass)
- [relative charge](https://www.gradenine.co.uk/glossary/relative-charge)

## 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/explain-how-the-relative-masses-and-ca26eddb) · Published by Druglandscape Ltd.