# Explain why the Geiger–Marsden alpha-particle scattering experiment led to the replacement of the Thomson ('plum pudding') model of the atom with the nuclear model.

> 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. Most particles passed straight through, but a small number were deflected through large angles, and a very few rebounded almost directly backwards.

## Mark scheme (4 marks)

1. The Thomson model predicts that positive charge is spread uniformly throughout the atom, so no region of sufficiently concentrated charge exists to cause large deflections.
2. The observation that most alpha particles pass through undeflected shows that most of the atom is empty space.
3. The small number of large-angle deflections (including backscattering) shows that the positive charge and most of the mass are concentrated in a very small, dense region — the nucleus.
4. The nuclear model — in which a tiny, massive, positively charged nucleus is surrounded by electrons orbiting in mostly empty space — is consistent with all the observations, whereas the Thomson model is not, so the Thomson model was rejected.

## Key terms

- [nuclear model](https://www.gradenine.co.uk/glossary/nuclear-model)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-geiger-marsden-alpha-particle-scattering-9e645786) · Published by Druglandscape Ltd.