Explain how the results of the gold foil experiment led Rutherford to propose the nuclear atomic model, and why these results could not be explained by Thomson's plum pudding model.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In the early twentieth century, Geiger and Marsden fired alpha particles at a thin sheet of gold foil. Most alpha particles passed straight through, but a small number were deflected at large angles, and a very few bounced almost straight back.
Model answer (4 marks)
Most alpha particles passed straight through the foil, showing that atoms are largely empty space.
A small number were deflected at large angles and a few were back‑scattered, which could only happen if a small, dense, positively charged nucleus was present at the centre of the atom.
In Thomson’s plum‑pudding model the positive charge is spread evenly throughout the atom, so the alpha particles would not experience the strong repulsive force needed for large‑angle deflections or back‑scattering.
Only a few alpha particles were deflected because the nucleus is very small compared with the size of the atom, so most particles miss it entirely.
A small number were deflected at large angles and a few were back‑scattered, which could only happen if a small, dense, positively charged nucleus was present at the centre of the atom.
In Thomson’s plum‑pudding model the positive charge is spread evenly throughout the atom, so the alpha particles would not experience the strong repulsive force needed for large‑angle deflections or back‑scattering.
Only a few alpha particles were deflected because the nucleus is very small compared with the size of the atom, so most particles miss it entirely.
Examiner tips
- Use the exact phrases: ‘mostly empty space’, ‘small, dense, positively charged nucleus’, ‘plum‑pudding model’, ‘back‑scattering’.
- Show the logical link: empty space → deflection → nucleus. Keep answer to 4 points.
Mark scheme (4 marks)
- Most alpha particles passing straight through shows that atoms are mostly empty space
- The large-angle deflections / backscattering show that there is a small, dense, positively charged nucleus at the centre of the atom
- In Thomson's plum pudding model the positive charge is spread evenly throughout the atom, so alpha particles would not be deflected at large angles / could not be deflected back
- Only a small number of alpha particles are deflected because the nucleus is very small compared to the size of the atom / most alpha particles miss the nucleus
Key terms in this question
nuclear atomic model · plum pudding model
Related
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