4.1.6Atomic Structure
The Plum Pudding Model — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 9 parts Lesson 7 of 23
Part 1 of 7Atomic models before 1900
1Dalton's solid sphere
- Dalton's solid sphere At the start of the 19th century atoms were pictured as tiny, solid, indivisible spheres, different for each element.
2Discovery of the electron
- Discovery of the electron In 1897 J.
- J.
- Thomson found that atoms contain tiny negatively charged particles, so atoms must have structure.
3Plum pudding model
- Plum pudding model Thomson proposed that the atom is a ball of positive charge with negative electrons embedded in it.
4Rutherford's evidence
- Rutherford's evidence Experiments with alpha particles showed the model to be wrong, leading to the nuclear model.
Part 2 of 7Features of the plum pudding model
Positive 'dough'
- Positive 'dough' A sphere of positive charge spread evenly throughout the atom.
Electrons as 'plums'
- Electrons as 'plums' Negatively charged electrons are scattered inside the sphere.
Neutral overall
- Neutral overall The total positive charge equals the total negative charge, so the atom has no overall charge.
No nucleus
- No nucleus The mass and positive charge are spread out; there is no tiny dense centre.
Part 3 of 7Why models change
1
A model is proposed to explain the evidence available at the time.
2
New experimental results are collected that the model cannot explain.
3
The model is modified or replaced by one that fits all the evidence.
4
The scientific community checks the new model by repeating experiments and testing predictions.
Part 4 of 7Comparing the models
| Feature | Plum pudding | Nuclear model |
|---|---|---|
| Positive charge | Spread through the whole atom | Concentrated in a tiny nucleus |
| Electrons | Embedded inside the sphere | Orbit around the nucleus |
| Mostly empty space? | No | Yes |
| Alpha scattering prediction | Small deflections only | Some large deflections, few reversed |
Part 5 of 7Timeline of atomic models
| Scientist | Approximate date | Contribution |
|---|---|---|
| John Dalton | Early 1800s | Atoms are solid spheres, with a different type for each element |
| J. J. Thomson | 1897–1904 | Discovered the electron and proposed the plum pudding model |
| Ernest Rutherford | 1909–1911 | Scattering results led to the nuclear model with a small positive nucleus |
| Niels Bohr | 1913 | Electrons orbit the nucleus in fixed energy levels or shells |
| James Chadwick | 1932 | Discovered the neutron in the nucleus |
Part 6 of 7How science develops
Scientific models
Evidence first
- Evidence first A model is only accepted while it explains all of the observations;
- the plum pudding model failed when new data appeared.
Peer review
- Peer review Other scientists check the methods and results, and repeat the experiments, before a new idea is widely accepted.
Models are useful
- Models are useful Older models are not stupid: they fitted the evidence at the time and are often still used to teach simple ideas.
Part 7 of 7Strengths and limits of the model
What it explained
- What it explained It accounted for electrons being present in atoms and for atoms being electrically neutral,
- which Dalton's solid sphere could not do.
What it could not explain
- What it could not explain It could not account for the large-angle scattering of alpha particles seen in 1909,
- because a diffuse positive charge cannot repel a fast alpha particle strongly enough.
Key terms
| Term | Meaning |
|---|---|
| Plum pudding model | An early atomic model with electrons embedded in a sphere of positive charge. |
| Electron | A negatively charged subatomic particle discovered by Thomson. |
| Model | A simplified representation used to explain observations and make predictions. |
| Evidence | Experimental results used to test or support a theory. |
| Nuclear model | The model in which most of the mass and all of the positive charge is in a small nucleus. |
| Peer review | The checking of scientific work by other experts before it is accepted. |
Avoid these mistakes
1
Placing a nucleus in the plum pudding model; it had no nucleus.
2
Writing that Thomson discovered the nucleus or the proton; he discovered the electron.
3
Saying the plum pudding model was 'wrong from the start' — it was a reasonable model for the evidence available.
4
Forgetting that the electrons carry negative charge and the 'pudding' carries positive charge.
Last updated: