Explain why the nuclear model of the atom replaced the plum pudding model.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Early scientists believed atoms were solid spheres of positive charge with electrons embedded throughout. Later experiments led to a new model being accepted by the scientific community.
Model answer (5 marks)
In the plum pudding model, the atom was thought to be a diffuse cloud of positive charge with electrons embedded throughout, so alpha particles would be expected to pass straight through with little or no deflection. In Rutherford’s scattering experiment, most alpha particles did indeed pass straight through, but a small number were deflected through large angles and some were even reflected back. Such large deflections could not be explained by a diffuse positive charge; they required a concentrated, positively charged centre. Therefore the nuclear model was adopted, proposing a small, dense nucleus at the centre of the atom, which accounts for the large-angle deflections of alpha particles.
Examiner tips
- Mention the key experimental observation (large-angle deflections) first to show understanding of why the plum pudding model failed.
- Explain how the nuclear model resolves this observation – a small, dense nucleus provides the required repulsive force.
Common mistakes
- Saying the plum pudding model predicted no deflection at all, ignoring that some deflection was expected.
- Confusing the nuclear model with the Bohr model – the nuclear model only describes the central charge, not electron orbits.
Mark scheme (5 marks)
- In the plum pudding model, alpha particles were expected to pass straight through (with little or no deflection)
- In the alpha particle scattering experiment, most alpha particles did pass straight through
- A small number of alpha particles were deflected through large angles / some were deflected back
- Large deflections could not be explained by the plum pudding model (so the model had to be changed)
- The nuclear model proposed a small, dense, positively charged nucleus at the centre of the atom, which explains why some alpha particles were deflected through large angles
Key terms in this question
plum pudding model · nuclear model
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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