What you'll learn
Our picture of the atom has changed dramatically over the last two hundred years, and each change came from new experimental evidence. For AQA GCSE Chemistry you need to know the sequence of models — Dalton's solid sphere, Thomson's plum pudding, Rutherford's nuclear model, Bohr's shells, and the discovery of the neutron — and explain what evidence caused each change. This guide works through the scientists in order, explains the alpha scattering experiment in detail, and shows how the modern model of the atom was built. By the end you should be able to describe each model, state who proposed it, and explain how experimental evidence led scientists to replace one model with the next.
Key terms and definitions
Atom — The smallest particle of an element that can exist.
Model — A simplified representation used to explain and predict; scientific models change when new evidence appears.
Plum pudding model — Thomson's model showing the atom as a ball of positive charge with negative electrons embedded in it.
Nucleus — The tiny, dense, positively charged centre of an atom, containing protons and neutrons.
Alpha particle — A positively charged particle (a helium nucleus) used in the scattering experiment.
Alpha scattering experiment — The experiment in which alpha particles were fired at thin gold foil, providing evidence for the nucleus.
Energy level (shell) — A fixed distance from the nucleus at which electrons orbit, proposed by Bohr.
Peer review — The checking of scientific work by other scientists, which is how new models come to be accepted.
Core concepts
Dalton's model (early 1800s)
John Dalton described atoms as tiny solid spheres that could not be divided or broken up. He proposed that each element was made of its own type of atom, and that atoms of different elements had different masses. At this time nothing was known about the inside of the atom — it was simply a solid ball. This model explained a lot about how elements combined, but it could not explain electricity or the structure inside the atom.
Thomson's model (1897) — the plum pudding
J. J. Thomson discovered the electron, a tiny negatively charged particle much smaller than an atom. This proved atoms were not solid spheres after all, because they contained smaller parts. Thomson proposed the plum pudding model: the atom was a ball of positive charge with negative electrons dotted through it, like fruit in a pudding. This model explained why atoms are neutral overall — the negative electrons balanced the positive charge.
Rutherford's alpha scattering experiment (1909)
Rutherford, together with Geiger and Marsden, tested the plum pudding model by firing positively charged alpha particles at a very thin sheet of gold foil. If the plum pudding model were correct, the positive charge would be spread thinly throughout the atom, and all the alpha particles should have passed straight through with only slight deflection.
The results were surprising:
- Most alpha particles passed straight through the foil.
- Some were deflected through small angles.
- A very few bounced almost straight back.
These results could not be explained by the plum pudding model.
What the results told scientists
Each observation led to a conclusion:
| Observation | Conclusion |
|---|---|
| Most passed straight through | The atom is mostly empty space |
| Some were deflected | The centre has a positive charge that repels alpha particles |
| A very few bounced back | The centre is very small but contains most of the mass |
Rutherford therefore proposed the nuclear model: the atom has a tiny, dense, positively charged nucleus at its centre, with electrons orbiting around it and mostly empty space in between. The plum pudding model was replaced.
Bohr's model (1913)
Rutherford's model had a problem: orbiting electrons should have spiralled into the nucleus, making atoms unstable. Niels Bohr solved this by proposing that electrons orbit the nucleus at fixed distances, in specific energy levels or shells, rather than anywhere. Bohr's calculations agreed with experimental observations, so this model was accepted. It is the model of shells still used at GCSE.
Protons and neutrons
Later experiments showed that the positive charge of the nucleus was made of individual particles, called protons, each with the same amount of positive charge. About twenty years after Bohr, James Chadwick provided evidence for the neutron — a particle in the nucleus with mass but no charge. This explained why the mass of a nucleus was greater than the mass of its protons alone, and completed the model of the atom used today.
The size of the atom and the nucleus
One striking result of Rutherford's work is just how empty an atom is. The nucleus is extremely small compared with the whole atom — if an atom were the size of a sports stadium, the nucleus would be about the size of a pea in the centre. Almost all of an atom's mass is packed into this tiny nucleus, while the electrons occupy the huge volume of mostly empty space around it. This is why most alpha particles passed straight through the gold foil: they simply travelled through the empty space and never came close to a nucleus. Understanding this scale helps explain all three scattering observations at once.
How and why scientific models change
This topic is a good example of how science works. A model is accepted only while it fits the evidence. When an experiment produces results the model cannot explain — as with alpha scattering — scientists propose a new model, and other scientists check it through peer review and further experiments. Models are therefore always provisional, and can be improved or replaced when better evidence appears. The story of the atom shows this clearly: five different scientists each refined the model as new evidence became available, and none of them was simply "wrong" — each built on the work before them.
Worked examples
Example 1: Explaining a scattering result
A very small number of alpha particles bounced straight back off the gold foil. What does this tell us about the atom? It shows the atom has a very small, dense, positively charged nucleus. The alpha particles are positive, so they were repelled strongly when they came close to a concentrated positive charge containing most of the atom's mass.
Example 2: Why the plum pudding model was rejected
Explain why the alpha scattering experiment led to the plum pudding model being replaced. The plum pudding model predicted the positive charge was spread thinly, so all alpha particles should have passed through with little deflection. Because some were deflected and a few bounced back, the positive charge must instead be concentrated in a tiny nucleus. The evidence did not fit the model, so it was replaced by the nuclear model.
Example 3: Putting the models in order
Place these in the correct order: Bohr's shells, Dalton's solid spheres, Rutherford's nuclear model, Thomson's plum pudding. The correct order is: Dalton's solid spheres, Thomson's plum pudding, Rutherford's nuclear model, then Bohr's shells. Each was replaced or refined as new experimental evidence appeared.
Example 4: The contribution of Chadwick
State what Chadwick discovered and why it mattered. Chadwick provided evidence for the neutron, a neutral particle in the nucleus. This mattered because it explained why the mass of the nucleus was greater than could be accounted for by protons alone.
Common mistakes and how to avoid them
A very common error is muddling who proposed what. Learn the pairings: Dalton — solid sphere; Thomson — plum pudding and the electron; Rutherford — nucleus and alpha scattering; Bohr — electron shells; Chadwick — neutron.
Students often state the alpha scattering observations without giving the conclusions. Each observation must be linked to what it shows: "most passed through, so the atom is mostly empty space". Marks are given for the link, not just the observation.
Another mistake is saying alpha particles were "attracted" by the nucleus. They are positive and the nucleus is positive, so they were repelled.
Be careful not to say the plum pudding model was "wrong because it was old". It was rejected because specific experimental evidence contradicted its prediction — always cite the evidence.
Finally, remember Bohr's contribution was that electrons orbit at fixed distances in energy levels, not simply "electrons orbit the nucleus", which was already Rutherford's idea.
Exam technique for "Development of the atomic model"
Questions on this topic frequently ask you to describe how the model changed and why. Always pair each model with the evidence that led to it — a description alone rarely gets full marks.
For alpha scattering, use a three-part structure: state the observation, state the conclusion, and name the model it supported. A table in your revision notes helps you memorise all three observations together.
This topic is also popular for "how science works" questions about why scientific models change. The key points are: models are based on evidence; when new evidence cannot be explained, the model is changed or replaced; and new models are checked by other scientists before being accepted. Practise writing the whole story of the atom in chronological order as a single paragraph.
Quick revision summary
- Dalton: atoms are tiny solid spheres that cannot be divided.
- Thomson: discovered the electron; proposed the plum pudding model (positive ball with electrons in it).
- Rutherford: alpha scattering showed most particles passed through (mostly empty space), some deflected (positive centre), a few bounced back (tiny dense nucleus) — giving the nuclear model.
- Bohr: electrons orbit the nucleus at fixed distances in energy levels (shells).
- Chadwick: provided evidence for the neutron, explaining the extra nuclear mass.
- Scientific models change when new experimental evidence cannot be explained by the current model.