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AQA GCSE Physics · Topic 4.4 Atomic structure
The Development of the Model of the Atom
In this lesson you'll learn to: put the five models of the atom in order, from Dalton's
solid sphere to Chadwick's neutron; describe the differences between the plum
pudding and nuclear models; explain how the alpha particle scattering results led to
the nuclear model; and explain why a scientific model is changed when new evidence doesn't
support it.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — put the models in order
Don't read anything yet. Below are five models of the atom, shuffled. Tap them into the timeline, oldest first, and press Check my order. Then comes the important part: for each change of model, pick the new evidence that forced it.
The model of the atom — put it in order
Step 1 of 2 · tap the models into the timeline, oldest first
Timeline (oldest → newest)
Models to place
Step 2 · what new evidence changed the model each time?
The rule: a model is only kept while the evidence supports it. When new evidence
didn’t support the model, scientists changed the model.
▲ Every change happened for the same reason: the new evidence didn't support the old model.
Now see the evidence for yourself
This is the experiment that killed the plum pudding model. Press Start the beam and watch the three tallies underneath. Then switch to One atom, close up and aim a single alpha particle at the nucleus: aim wide, then aim closer and closer.
Rutherford scattering — firing alpha particles at gold foil
0Most pass straight through
so the atom must be mostly empty space
0Some are deflected
so the centre must be positively charged, repelling the alpha
0A very few bounce back
so the centre is tiny and holds most of the mass
▲ Most go straight through, some are deflected, and a very few bounce straight back.
Got the order? Now learn what each model said.
The exam asks you to describe the models, compare plum pudding with the nuclear model, and explain how the scattering results changed it. The notes below give you all three.
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Revision notes
1. Why the model keeps changing
Nobody can see inside an atom, so scientists build a model: a description of what they think it is like, based on the evidence they have. A model is only kept while the evidence supports it.
⭐ Memorise this
When new evidence doesn't agree with a model, scientists change the model, or replace it with a new one.
2. The five models, in order
1 · Dalton
early 1800sAtoms are tiny solid spheres that cannot be divided.
2 · Thomson
1897Discovered the electron. Plum pudding: a ball of positive charge with electrons embedded in it.
3 · Rutherford
1911Alpha scattering: mass and positive charge are in a tiny nucleus; most of the atom is empty space.
4 · Bohr
1913Electrons orbit at fixed distances, in shells (energy levels). His calculations agreed with experiments.
5 · Chadwick
1932Discovered the neutron in the nucleus, about 20 years after the nucleus was accepted.
Between Bohr and Chadwick, later experiments showed that the positive charge of any nucleus could be divided into a whole number of smaller particles, each with the same charge. These became known as protons.
3. Plum pudding vs the nuclear model
This comparison is the most common exam question on the topic. Learn it side by side:
| Plum pudding model | Nuclear model | |
|---|---|---|
| Positive charge | spread uniformly through the whole atom | concentrated in a tiny central nucleus |
| Mass | spread through the whole atom | nearly all in the nucleus |
| Electrons | embedded in the ball of positive charge | negatively charged, orbiting outside the nucleus |
| Empty space? | no — a solid ball of charge | mostly empty space |
4. The alpha particle scattering experiment
Rutherford's team fired alpha particles (small, positive and fast) at a very thin sheet of gold foil, and recorded where they went. The plum pudding model predicted that every one would pass straight through, because its positive charge was spread too thinly to push them off course.
Each observation told them one thing about the atom. The marks are for linking each observation to its conclusion:
| What they saw (evidence) | So the atom must be… (model) |
|---|---|
| Most alpha particles passed straight through | mostly empty space |
| Some were deflected | the centre is positively charged, repelling the positive alpha particles |
| A very few bounced back | almost all the mass is concentrated in a tiny nucleus |
💡 Exam tip — join each pair with "so"
A list of three observations and three conclusions that are never linked is capped. Write "most went straight through, so the atom is mostly empty space", and do the same for all three.
5. Bohr and Chadwick: adapting the nuclear model
The nuclear model wasn't the end. Niels Bohr suggested that electrons orbit the nucleus at specific distances, in shells or energy levels. His theoretical calculations agreed with experimental observations, so the model was adapted. About 20 years after the nucleus was accepted, James Chadwick's experiments provided the evidence for the neutron, and the nucleus became protons and neutrons.
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How to write each answer
These questions come up again and again. These are the sentences that get the marks — learn the shape, then swap in the details from your question.
“Describe the differences between the plum pudding model and the nuclear model.”
In the plum pudding model the positive charge is spread
uniformly through the atom, with the electrons embedded in it. In the
nuclear model the positive charge and mass are concentrated in a tiny nucleus,
the negative electrons orbit outside it, and most of the atom is
empty space.
“Explain why the plum pudding model was replaced.”
The plum pudding model predicted that all the alpha particles would pass straight
through. The evidence showed a few bounced back,
which disagreed with the prediction, so scientists had to
change the model.
“Link each observation to its conclusion.”
Most passed straight through, so the atom is mostly empty
space. Some were deflected, so the centre is positively
charged. A very few bounced back, so the mass is
concentrated in a tiny nucleus.
the evidence or model what it shows the conclusion
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Worked examples
These are the questions from pages 4 and 5 of the workbook. Cover the answer, try it, then check.
Example 1 — plum pudding vs nuclear
Describe the differences between the plum pudding model and the nuclear model of the atom.4 marks
1. In the plum pudding model, the positive charge is spread uniformly through the whole atom.
2. In the plum pudding model, the electrons are embedded inside that ball of positive charge.
3. In the nuclear model, the positive charge and nearly all the mass are concentrated in a tiny central nucleus.
4. In the nuclear model, the negatively charged electrons orbit outside the nucleus, and most of the atom is empty space.
2. In the plum pudding model, the electrons are embedded inside that ball of positive charge.
3. In the nuclear model, the positive charge and nearly all the mass are concentrated in a tiny central nucleus.
4. In the nuclear model, the negatively charged electrons orbit outside the nucleus, and most of the atom is empty space.
Mark scheme: one mark for each point. You need the words uniform, positive, electrons, nucleus and negatively.
Example 2 — why the model changed
In the alpha particle scattering experiment, a few alpha particles bounced straight back off the gold foil. Explain why this result meant the plum pudding model had to be replaced.4 marks
1. The plum pudding model predicted every alpha particle would pass straight through, because the positive charge was spread thinly.
2. The data from the experiment showed a few particles bounced straight back: this was new evidence.
3. This result disagreed with the prediction, so the model could not be kept.
4. So scientists had to change the model, to a tiny, dense, positively charged nucleus that could repel an alpha particle backwards.
2. The data from the experiment showed a few particles bounced straight back: this was new evidence.
3. This result disagreed with the prediction, so the model could not be kept.
4. So scientists had to change the model, to a tiny, dense, positively charged nucleus that could repel an alpha particle backwards.
Tip: this is the "how science works" answer. A model survives only while it agrees with the evidence.
Example 3 — the exam-style question
Alpha particles were fired at a very thin sheet of gold foil. Explain how the results of this experiment led scientists to the nuclear model of the atom. In your answer, link each observation to the conclusion it supports.5 marks
Observation: most of the alpha particles passed straight through the foil,
so the atom is mostly empty space.
Observation: some were deflected, so the centre of the atom is positively charged, repelling the positive alpha particles.
Observation: a very few bounced straight back, so almost all the mass is concentrated in a tiny central nucleus.
Observation: some were deflected, so the centre of the atom is positively charged, repelling the positive alpha particles.
Observation: a very few bounced straight back, so almost all the mass is concentrated in a tiny central nucleus.
Mark scheme: any five of those six points. The marks are for the links, not the lists: look for "so", "this shows" or "because" between each pair.
📘 Now do it in your workbook
Draw each model on the timeline, compare plum pudding with the nuclear model, then link each scattering observation to its conclusion — with worked answers at the back so you can mark your own.
AQA GCSE Physics Workbook · ATOMIC STRUCTURE · pages 4 & 5Free sample = the pages for this lesson. The full workbook covers the whole of Topic 4 Atomic Structure, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the notes and the free workbook pages above cover every mark.
Now put it into practice
Watching is the easy bit — the links between evidence and model only stick once you've written them yourself. Pages 4 and 5 of the workbook are free, and the full book covers every spec point in Topic 4 Atomic Structure with worked answers.
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