Explain why the results of the Geiger–Marsden (gold foil) experiment led to the replacement of the Thomson 'plum pudding' model with the nuclear 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 very thin gold foil. Most alpha particles passed straight through, a small number were deflected at small angles, and a very small number were deflected back through angles greater than 90°.
Model answer (4 marks)
The Thomson model assumed a uniform, diffuse distribution of positive charge throughout the atom, so it could not explain the large‑angle deflections observed.
The fact that most alpha particles passed straight through the foil shows that the atom is largely empty space.
The few alpha particles that were deflected at large angles or back‑scattered reveal that the positive charge is concentrated in a very small, dense region – the nucleus.
Therefore the nuclear model was proposed, placing almost all the atom’s mass and positive charge in a tiny central nucleus, with electrons orbiting in the surrounding empty space.
The fact that most alpha particles passed straight through the foil shows that the atom is largely empty space.
The few alpha particles that were deflected at large angles or back‑scattered reveal that the positive charge is concentrated in a very small, dense region – the nucleus.
Therefore the nuclear model was proposed, placing almost all the atom’s mass and positive charge in a tiny central nucleus, with electrons orbiting in the surrounding empty space.
Examiner tips
- Use the exact phrases from the mark scheme (e.g. ‘large‑angle deflections’, ‘tiny, dense region’, ‘nucleus’).
- Show the logical progression: Thomson model → experimental evidence → nuclear model.
- Keep the answer concise, 4 points, no extra words.
Common mistakes
- Confusing the Thomson model with the Rutherford model; writing that the Thomson model had a nucleus. Failing to mention that most alpha particles passed straight through, indicating empty space. Using vague terms like ‘small region’ without specifying ‘tiny, dense nucleus’.
Mark scheme (4 marks)
- The Thomson model predicted a uniform, diffuse distribution of positive charge, so it could not account for large-angle deflections of alpha particles.
- Most alpha particles passing straight through shows that most of the atom is empty space.
- The small number of large-angle / back-scattering deflections shows that positive charge is concentrated in a very small, dense region called the nucleus.
- The nuclear model therefore proposes that almost all the atom's mass and all its positive charge are concentrated in a tiny central nucleus, with electrons occupying the surrounding (largely empty) space.
Key terms in this question
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More The nuclear atom questions
- Explain why atoms of the same element can have different masses, and outline how…
- Explain why the atomic number of an element uniquely identifies that element, an…
- Explain why the mass number of an atom is always a whole number, yet the relativ…
- Explain why two atoms of the same element may behave identically in chemical rea…