Section 1 of 7Why models change
Models explain observations
- Models explain observations A scientific model is an idea used to explain what is seen in experiments.
- The best model is the one that explains the widest range of evidence.
Evidence decides
- Evidence decides If new results do not fit a model, the model is modified or replaced.
- This is why ideas about the atom have changed several times.
Prediction and testing
- Prediction and testing Scientists use a model to make predictions, test them, and keep the model only while the results agree.
Section 2 of 7How the atom was pictured
1Solid ball
- Solid ball The earliest idea was that atoms are tiny, solid spheres that cannot be divided.
2Plum pudding
- Plum pudding After the electron was found, the atom was described as a ball of positive charge with negative electrons embedded in it.
3Nuclear model
- Nuclear model Alpha scattering results showed a tiny, dense, positive nucleus with electrons around it.
4Bohr and beyond
- Bohr and beyond Electrons were placed in energy levels, then the proton and neutron were identified inside the nucleus.
Section 3 of 7Plum pudding and nuclear models compared
| Feature | Plum pudding model | Nuclear model |
|---|---|---|
| Positive charge | Spread evenly through the whole atom | Concentrated in a tiny nucleus |
| Electrons | Embedded within the positive material | Outside the nucleus, in the space around it |
| Mass | Spread out through the atom | Almost all in the nucleus |
| Alpha particles fired at gold foil | Predicts only tiny deflections | Explains rare large deflections and bounce-backs |
Section 4 of 7The alpha scattering experiment
1
Alpha particles were fired at a very thin sheet of gold foil.
2
If the plum pudding model were correct, nearly all particles would pass through with, at most, slight deflections.
3
Most particles did go straight through, but a very small fraction, about 1 in 8000, were deflected through large angles and some bounced almost straight back.
4
These results could not be explained by the plum pudding model, so it was rejected.
5
The nuclear model explained them: a concentrated positive nucleus repels the few alpha particles that pass close to it.
Section 5 of 7Why matching predictions matters
Evidence and prediction
Rutherford's calculation
- Rutherford's calculation Using the nuclear model, Rutherford worked out the proportion of alpha particles expected at different angles.
Agreement with results
- Agreement with results The measured results matched his predictions, which gave strong support to the nuclear model.
Still open to change
- Still open to change Support is not final proof;
- later evidence led to the Bohr model and to the discovery of neutrons.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Plum pudding model | An early model with electrons scattered through a ball of positive charge. |
| Nuclear model | A model with a small, dense, positively charged nucleus surrounded by electrons. |
| Alpha scattering | The experiment in which alpha particles were fired at thin gold foil and their deflections were measured. |
| Prediction | A statement about what should be observed if a model is correct. |
| Evidence | Experimental results used to support or reject a model. |
Section 7 of 7Exam technique
1
Saying most alpha particles bounced back - most passed straight through; only a tiny number were deflected strongly.
2
Claiming the plum pudding model was wrong because nobody believed it; it was rejected because it could not explain the experimental results.
3
Describing the nucleus as large - it is very small compared with the whole atom, yet holds almost all the mass.
4
Saying that agreement between prediction and result 'proves' a model beyond doubt; it supports the model but it can still be refined.