Explain why the results of the Geiger–Marsden alpha-particle scattering experiment could not be accounted for by the Thomson ('plum-pudding') model of the atom.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In the Geiger–Marsden experiment, a beam of alpha particles was directed at a thin gold foil. Most alpha particles passed straight through, but a small fraction were deflected through large angles, and a very small number were scattered back towards the source.
Model answer (4 marks)
The Thomson model assumes the positive charge is spread uniformly throughout the atom. Consequently the electric field acting on an incoming alpha particle is weak and extends over a large distance, giving only small deflections. The experiment, however, shows that a few alpha particles are scattered through large angles, even back‑scattered, which requires a very strong, concentrated repulsive force. This cannot be produced by a diffuse positive charge, so the Thomson model cannot explain the results.
Examiner tips
- Use the key point that the Thomson model predicts diffuse charge → small deflections; contrast with large-angle scattering.
- Mention the need for a concentrated positive charge (nucleus).
Common mistakes
- Claiming the Thomson model predicts large deflections; it predicts the opposite.
- Forgetting that back‑scattering requires a very strong, central force.
Mark scheme (4 marks)
- The Thomson model predicts a diffuse, uniform distribution of positive charge throughout the atom, so the electric force on any alpha particle would be small and spread over a large distance.
- Therefore, the Thomson model predicts only very small deflections for all alpha particles, not large-angle or back-scattering.
- The observation that a small fraction of alpha particles are scattered through large angles (greater than 90°) shows that a very strong, concentrated repulsive force must exist within the atom.
- This evidence led to the nuclear model in which almost all the positive charge (and mass) is concentrated in a tiny, dense nucleus, which the Thomson model cannot explain.
Key terms in this question
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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