# Explain why the results of the Geiger–Marsden (gold foil) experiment led to the replacement of the Thomson 'plum pudding' model with the nuclear model of the atom.

> IB DP Chemistry Standard Level (2023 syllabus) — S1.2 The nuclear atom · Explain · 4 marks

> In the Geiger–Marsden experiment, a beam of alpha particles was directed at a very thin gold foil. Most alpha particles passed straight through, a small number were deflected at small angles, and a very small number were deflected back through angles greater than 90°.

## Mark scheme (4 marks)

1. The Thomson model predicted a uniform, diffuse distribution of positive charge, so it could not account for large-angle deflections of alpha particles.
2. Most alpha particles passing straight through shows that most of the atom is empty space.
3. The small number of large-angle / back-scattering deflections shows that positive charge is concentrated in a very small, dense region called the nucleus.
4. The nuclear model therefore proposes that almost all the atom's mass and all its positive charge are concentrated in a tiny central nucleus, with electrons occupying the surrounding (largely empty) space.

## Key terms

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

## Related

- [Revision notes for IB DP Chemistry 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-results-of-the-5f79da61) · Published by Druglandscape Ltd.