# Explain how the atomic model changed after the discovery of the nucleus, and describe the evidence from the alpha particle scattering experiment that led to this change.

> AQA A-Level Chemistry (7405) — 3.1.1 Atomic structure · Explain · 5 marks

> Before 1911, scientists used the plum pudding model to describe the atom. This model was later replaced following experimental evidence collected by Geiger and Marsden.

## Mark scheme (5 marks)

1. In the plum pudding model, the atom was described as a ball of positive charge with electrons embedded in it
2. Most alpha particles passed straight through the gold foil, showing the atom is mostly empty space
3. A small number of alpha particles were deflected at large angles / some bounced back
4. The large-angle deflections showed that most of the mass / positive charge is concentrated in a small, dense nucleus
5. The nuclear model replaced the plum pudding model because the plum pudding model could not explain the large-angle deflections

## Key terms

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

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](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-atomic-model-changed-4b630bd7) · Published by Druglandscape Ltd.