Explain how the observation that most alpha particles passed straight through gold foil without deflection, while a very small number were deflected through large angles, supports 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 thin gold foil and a detector was used to count particles arriving at different angles.
Model answer (4 marks)
Most alpha particles pass straight through because the atom is largely empty space; the electron cloud does not significantly deflect them.
Only a very small number of particles come close to the tiny nucleus, which is very small compared with the atom.
When an alpha particle does approach the nucleus, the concentrated positive charge produces a strong repulsive Coulomb force, deflecting it through a large angle.
This behaviour cannot be explained by the Thomson model, where charge is spread uniformly; the observed large deflections therefore support the nuclear model of the atom.
Only a very small number of particles come close to the tiny nucleus, which is very small compared with the atom.
When an alpha particle does approach the nucleus, the concentrated positive charge produces a strong repulsive Coulomb force, deflecting it through a large angle.
This behaviour cannot be explained by the Thomson model, where charge is spread uniformly; the observed large deflections therefore support the nuclear model of the atom.
Examiner tips
- Use the word ‘deflect’ and mention Coulomb force for large angles.
- Show that most of the atom is empty space to justify straight‑through particles.
- Contrast with the Thomson model explicitly to gain the mark for inconsistency.
- Keep the answer to four concise points, matching the mark scheme.
Mark scheme (4 marks)
- Most alpha particles pass straight through because most of the atom is empty space (the atom is mostly empty / the electron cloud does not significantly deflect alpha particles).
- The nucleus is very small compared to the size of the atom, so very few alpha particles come close to it.
- Large-angle deflections occur because the nucleus contains most of the mass / positive charge concentrated in a very small volume, producing a strong repulsive (electrostatic / Coulomb) force on the alpha particle.
- This is inconsistent with the Thomson model (plum-pudding model), in which charge is spread uniformly throughout the atom and could not produce the observed large deflections, so the nuclear model (concentrated nucleus) is required.
Key terms in this question
alpha particle · nuclear model
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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