Explain why the Geiger–Marsden alpha-particle scattering experiment led to the replacement of the Thomson ('plum pudding') model of the atom with the nuclear model.

IB DP Physics Standard Level (2023 syllabus) — E.1 Structure of the atom · Explain · 4 marks · View as Markdown

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 particles passed straight through, but a small number were deflected through large angles, and a very few rebounded almost directly backwards.

Model answer (4 marks)

The Thomson model predicts a uniform distribution of positive charge throughout the atom, so no concentrated region of charge exists to deflect alpha particles by large angles.

The experiment shows that most alpha particles pass through the foil undeflected, indicating that the atom is largely empty space.

A few particles are deflected through very large angles, including backscattering, which can only occur if a very small, dense region of positive charge and mass exists.

Therefore the nuclear model, with a tiny, massive, positively charged nucleus surrounded by electrons in mostly empty space, explains all observations, whereas the Thomson model cannot.

Examiner tips

  • Use the word ‘because’ to link observations to conclusions; mention ‘empty space’ and ‘dense nucleus’ explicitly.
  • Show the logical progression: uniform charge → no deflection, empty space → most particles pass, dense nucleus → large deflections.
  • Keep each point short and to the point to fit the 4‑mark limit.

Common mistakes

  • Failing to mention that the Thomson model predicts no large deflections; or not stating that the nucleus is tiny and dense.
  • Over‑expanding the answer with unnecessary detail or repeating the same idea twice.
  • Using vague terms like ‘positive charge’ without specifying ‘concentrated in a small region’.

Mark scheme (4 marks)

  1. The Thomson model predicts that positive charge is spread uniformly throughout the atom, so no region of sufficiently concentrated charge exists to cause large deflections.
  2. The observation that most alpha particles pass through undeflected shows that most of the atom is empty space.
  3. The small number of large-angle deflections (including backscattering) shows that the positive charge and most of the mass are concentrated in a very small, dense region — the nucleus.
  4. The nuclear model — in which a tiny, massive, positively charged nucleus is surrounded by electrons orbiting in mostly empty space — is consistent with all the observations, whereas the Thomson model is not, so the Thomson model was rejected.

Key terms in this question

nuclear model

Related

More Structure of the atom questions

▶ Try answering this question with AI marking (free) →