Describe how scientists' understanding of atomic structure developed from Dalton's model to the nuclear model. In your answer, refer to the key experiments and discoveries that led to changes in the model.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Dalton’s model (1803) described atoms as solid, indivisible spheres – the ‘billiard‑ball’ model.
Thomson (1897) discovered the electron in cathode‑ray experiments, showing atoms contain smaller, negatively charged particles.
Thomson proposed the plum‑pudding model: electrons embedded in a diffuse sphere of positive charge.
Rutherford’s gold‑foil experiment (1911) fired alpha particles at thin gold foil. Most particles passed straight through, but a few were deflected at large angles or even rebounded.
These results led to the nuclear model: atoms have a small, dense, positively charged nucleus surrounded by electrons.
Thomson (1897) discovered the electron in cathode‑ray experiments, showing atoms contain smaller, negatively charged particles.
Thomson proposed the plum‑pudding model: electrons embedded in a diffuse sphere of positive charge.
Rutherford’s gold‑foil experiment (1911) fired alpha particles at thin gold foil. Most particles passed straight through, but a few were deflected at large angles or even rebounded.
These results led to the nuclear model: atoms have a small, dense, positively charged nucleus surrounded by electrons.
Examiner tips
- Use the exact terminology (billiard‑ball, plum‑pudding, nuclear model).
- Sequence the models chronologically and link each experiment to the change in model.
- Include the key experimental evidence (cathode rays, gold‑foil scattering).
Common mistakes
- Mixing up the order of the models or experiments.
- Failing to mention the deflection of alpha particles in Rutherford’s experiment.
Mark scheme (5 marks)
- Dalton proposed that atoms were solid, indivisible spheres (or 'billiard ball' model)
- Thomson discovered the electron (through cathode ray experiments), showing atoms contain smaller, negatively charged particles
- Thomson proposed the plum pudding model, in which electrons are embedded in a ball of positive charge
- Rutherford's alpha particle scattering experiment fired alpha particles at thin gold foil and found that most passed straight through
- A small number of alpha particles were deflected at large angles (or bounced back), leading to the conclusion that the atom has a small, dense, positively charged nucleus
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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