# Explain how the observation that most alpha particles passed straight through gold foil without deflection, while a very small number were deflected through large angles, supports the nuclear model of the atom.

> 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 and a detector was used to count particles arriving at different angles.

## Mark scheme (4 marks)

1. Most alpha particles pass straight through because most of the atom is empty space (the atom is mostly empty / the electron cloud does not significantly deflect alpha particles).
2. The nucleus is very small compared to the size of the atom, so very few alpha particles come close to it.
3. Large-angle deflections occur because the nucleus contains most of the mass / positive charge concentrated in a very small volume, producing a strong repulsive (electrostatic / Coulomb) force on the alpha particle.
4. This is inconsistent with the Thomson model (plum-pudding model), in which charge is spread uniformly throughout the atom and could not produce the observed large deflections, so the nuclear model (concentrated nucleus) is required.

## Key terms

- [alpha particle](https://www.gradenine.co.uk/glossary/alpha-particle)
- [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)
- [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-most-95f1b241) · Published by Druglandscape Ltd.