Explain why the results of the Geiger–Marsden (gold foil) experiment could not be accounted for by the Thomson 'plum pudding' model, and outline what the experimental observations revealed about the structure of the atom.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The plum‑pudding model predicted that alpha particles would pass straight through or be only slightly deflected, because the positive charge was assumed to be spread diffusely throughout the atom.
In the experiment a small fraction of the alpha particles were deflected through large angles (greater than 90°), and some even bounced almost straight back. This cannot be explained by a diffuse positive charge.
The large‑angle deflections showed that most of the positive charge (and mass) of the atom is concentrated in a very small, dense region – the nucleus.
The fact that most alpha particles passed straight through (were undeflected) demonstrated that the atom is mostly empty space, with electrons occupying the region outside the nucleus.
In the experiment a small fraction of the alpha particles were deflected through large angles (greater than 90°), and some even bounced almost straight back. This cannot be explained by a diffuse positive charge.
The large‑angle deflections showed that most of the positive charge (and mass) of the atom is concentrated in a very small, dense region – the nucleus.
The fact that most alpha particles passed straight through (were undeflected) demonstrated that the atom is mostly empty space, with electrons occupying the region outside the nucleus.
Examiner tips
- Use the exact wording ‘large angles >90°’ and ‘almost straight back’ to show understanding of the observations.
- Explain how the observations contradict the diffuse charge assumption before stating the nuclear model.
- Mention that the nucleus is small and dense to justify the deflections.
- Show the link between undeflected particles and empty space to complete the explanation.
Common mistakes
- Saying the plum‑pudding model predicted only small deflections without specifying the magnitude; examiners look for the >90° detail.
- Confusing the deflection of electrons with alpha particles; the experiment used alpha particles.
- Failing to mention that the nucleus is small and dense; just saying ‘positive charge concentrated’ is incomplete.
Mark scheme (4 marks)
- The plum pudding model predicted that alpha particles would pass straight through or be only slightly deflected, because positive charge was assumed to be spread diffusely throughout the atom.
- A small fraction of alpha particles were deflected through large angles (greater than 90°), including some that bounced almost straight back, which the plum pudding model cannot explain.
- The large-angle deflections showed that most of the positive charge (and mass) of the atom is concentrated in a very small, dense region called the nucleus.
- The fact that most alpha particles passed straight through (were undeflected) showed that the atom is mostly empty space, with electrons occupying the region outside the nucleus.
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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