Describe the evidence from the alpha particle scattering experiment that led scientists to conclude that an atom has a small, dense, positively charged nucleus.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Before the alpha particle scattering experiment, scientists used the plum pudding model to describe atomic structure. In this model, the atom was a sphere of positive charge with negative electrons embedded throughout.
Model answer (5 marks)
Most alpha particles passed straight through the gold foil with little or no deflection, showing that the atom is mostly empty space.
A small number of alpha particles were deflected through large angles or even back towards the source, indicating that the scattering centre is very small.
The large deflections show that this small centre is positively charged, because it repels the positively charged alpha particles.
The fact that only a few particles were deflected implies that the centre is tiny compared with the size of the atom.
The large deflections also show that the centre contains most of the atom’s mass, because it must be dense to produce such strong repulsion.
A small number of alpha particles were deflected through large angles or even back towards the source, indicating that the scattering centre is very small.
The large deflections show that this small centre is positively charged, because it repels the positively charged alpha particles.
The fact that only a few particles were deflected implies that the centre is tiny compared with the size of the atom.
The large deflections also show that the centre contains most of the atom’s mass, because it must be dense to produce such strong repulsion.
Examiner tips
- Use the exact wording from the mark scheme – e.g. ‘most alpha particles passed straight through’. Explain each point in one sentence to hit the 5 marks. Mention both size and charge of the nucleus as required.
Common mistakes
- Confusing the plum pudding model with the Rutherford model. Saying the nucleus is ‘large’ instead of ‘small’. Forgetting to link the deflections to the nucleus’s charge and density.
Mark scheme (5 marks)
- Most alpha particles passed straight through the gold foil (with little or no deflection)
- This shows that the atom is mostly empty space
- A small number of alpha particles were deflected through large angles / back towards the source
- This shows the nucleus is small (because so few particles were deflected)
- The large deflections show the nucleus is positively charged (because it repels the positively charged alpha particles) AND dense / contains most of the mass of the atom
Key terms in this question
alpha particle scattering experiment · nucleus
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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