# Explain how the observation that a small fraction of alpha particles in the Geiger–Marsden experiment were deflected through angles greater than 90° led to the conclusion that the positive charge of an atom is concentrated in a small, dense nucleus.

> IB DP Physics Standard Level (2023 syllabus) — E.1 Structure of the atom · Explain · 4 marks

> In the Geiger–Marsden experiment, a beam of alpha particles was directed at a thin gold foil. Whilst the vast majority passed through with little or no deflection, approximately 1 in 8000 alpha particles were deflected through angles greater than 90°.

## Mark scheme (4 marks)

1. Large-angle deflections require a large electrostatic (Coulomb) repulsive force acting on the alpha particle.
2. Such a large force is only possible if the positive charge is concentrated in a very small region (nucleus), producing an intense electric field at close range.
3. The rarity of large deflections indicates that the nucleus occupies only a tiny fraction of the atom's volume, so most alpha particles pass far from it and are undeflected.
4. The nucleus must also be dense (massive) because the alpha particle is brought almost to rest and reversed in direction, implying the nucleus does not recoil significantly.

## Key terms

- [alpha particle](https://www.gradenine.co.uk/glossary/alpha-particle)
- [nucleus](https://www.gradenine.co.uk/glossary/nucleus)

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](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-observation-that-a-74a8dc59) · Published by Druglandscape Ltd.