# Explain why the results of the Geiger–Marsden alpha-particle scattering experiment could not be accounted for by the Thomson ('plum-pudding') model of the atom.

> IB DP Physics Higher 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 alpha particles passed straight through, but a small fraction were deflected through large angles, and a very small number were scattered back towards the source.

## Mark scheme (4 marks)

1. The Thomson model predicts a diffuse, uniform distribution of positive charge throughout the atom, so the electric force on any alpha particle would be small and spread over a large distance.
2. Therefore, the Thomson model predicts only very small deflections for all alpha particles, not large-angle or back-scattering.
3. The observation that a small fraction of alpha particles are scattered through large angles (greater than 90°) shows that a very strong, concentrated repulsive force must exist within the atom.
4. This evidence led to the nuclear model in which almost all the positive charge (and mass) is concentrated in a tiny, dense nucleus, which the Thomson model cannot explain.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Higher 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-results-of-the-491572fd) · Published by Druglandscape Ltd.