# Describe how the nuclear model of the atom developed from the plum pudding model, following the results of the alpha particle scattering experiment.

> Edexcel A-Level Chemistry (9CH0) — 1.1 Atomic structure · Describe · 5 marks

> In 1909, Geiger and Marsden fired positively charged alpha particles at a very thin sheet of gold foil and detected where the particles went after hitting the foil. Most passed straight through, some were deflected by small angles, and a very small number bounced almost directly back.

## Mark scheme (5 marks)

1. The plum pudding model described the atom as a ball of positive charge with electrons embedded throughout it (like plums in a pudding)
2. The plum pudding model predicted all alpha particles should pass straight through or be only slightly deflected, so the large-angle deflections / back-scattering were unexpected
3. Most alpha particles passed straight through because most of the atom is empty space
4. A small, dense, positively charged nucleus was proposed to explain the large-angle deflections / back-scattering
5. In the nuclear model electrons orbit the nucleus at a distance / outside the nucleus

## Key terms

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

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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