Explain why scientists changed the plum pudding model of the atom after the results of the alpha particle scattering experiment were published.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In the early 1900s, Geiger and Marsden fired positively charged 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 small number bounced almost straight back.
Model answer (5 marks)
In the plum‑pudding model the positive charge was thought to be spread evenly throughout the atom, so large deflections of alpha particles were not expected.
The experiment showed that most alpha particles passed straight through the foil, indicating that most of the atom is empty space.
A small number of alpha particles were deflected at large angles and a very small number bounced back, proving that a small, dense, positively charged nucleus exists.
Because only a very small proportion of alpha particles were deflected or bounced back, the nucleus must be very small compared with the whole atom.
These findings contradicted the plum‑pudding model, leading scientists to adopt the nuclear model in which electrons orbit a central nucleus.
The experiment showed that most alpha particles passed straight through the foil, indicating that most of the atom is empty space.
A small number of alpha particles were deflected at large angles and a very small number bounced back, proving that a small, dense, positively charged nucleus exists.
Because only a very small proportion of alpha particles were deflected or bounced back, the nucleus must be very small compared with the whole atom.
These findings contradicted the plum‑pudding model, leading scientists to adopt the nuclear model in which electrons orbit a central nucleus.
Examiner tips
- Use the exact points from the mark scheme, in order; each point counts one mark.
- Show the logical sequence: expectation, observation, inference, conclusion.
- Keep sentences short and factual, no extra commentary.
Common mistakes
- Claiming the plum‑pudding model predicted large deflections; it predicted none.
- Forgetting to mention that the nucleus is small compared with the atom.
- Using vague terms like "changed the model" without specifying the nuclear model.
Mark scheme (5 marks)
- In the plum pudding model, positive charge is spread evenly throughout the atom, so large deflections of alpha particles were not expected
- Most alpha particles passing straight through shows that most of the atom is empty space
- Some alpha particles were deflected at large angles / bounced back, showing a small, dense, positively charged nucleus exists
- Only a very small proportion of alpha particles were deflected/bounced back, showing the nucleus is very small compared to the whole atom
- The evidence contradicted the plum pudding model, so scientists revised it to produce the nuclear model in which electrons orbit a central nucleus
Key terms in this question
plum pudding model · alpha particle scattering experiment
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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