The Nuclear Model Revision Notes | GCSE AQA Higher Physics
In Rutherford's alpha particle scattering experiment, alpha particles were fired at thin gold foil, and most passed straight through. Free notes and FAQs.
Updated 20 Mar 2026 · Revise Right Team
The Nuclear Model is part of Atoms and isotopes in GCSE AQA Higher Physics.
3 min
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What this lesson covers
In Rutherford's alpha particle scattering experiment, alpha particles were fired at thin gold foil, and most passed straight through. Free notes and FAQs.
Explain why the alpha particle scattering experiment led to a change in atomic model
Compare the plum pudding model with the nuclear model of the atom
Describe how new experimental evidence can lead to scientific models being changed or replaced
Evaluate the significance of Rutherford's conclusions about atomic structure
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Use this lesson to stay inside Atoms and isotopes while connecting the detail back to the wider Atomic structure topic.
Atomic structure
Atoms and isotopes
GCSE AQA Higher Physics
Revision notes
Read the core explanations from The Nuclear Model before testing yourself from memory.
1. The Problem with Atoms
By 1900, scientists knew atoms contained positive and negative charges, but nobody knew how they were arranged. Thomson's 'plum pudding' model suggested atoms were spheres of positive charge with electrons dotted throughout like plums in a pudding.
This model seemed reasonable - it explained why atoms were neutral overall. But in 1909, Ernest Rutherford designed an experiment that would shatter this comfortable picture forever.
2. Key Models of the Atom
Thomson proposed this model in 1904 after discovering electrons. He imagined the atom as a 'pudding' of positive charge with negative electrons scattered inside. The positive and negative charges balanced out, making the atom neutral.
Rutherford's nuclear model, proposed in 1911, was revolutionary. Instead of charge spread throughout the atom, almost all the mass and positive charge was squeezed into a tiny central nucleus. Electrons orbited this nucleus like planets around the sun.
3. The Alpha Particle Scattering Experiment
Rutherford fired alpha particles (helium nuclei with +2 charge) at extremely thin gold foil. If Thomson's plum pudding model was correct, the alpha particles should pass through with only slight deflections - like bullets through jelly.
The results were shocking. Most particles did pass straight through, but some were deflected at large angles. Most surprising of all, about 1 in 8000 particles bounced straight back towards the source.
4. Why the Evidence Changed Everything
Rutherford famously said the results were 'as if you fired a 15-inch shell at tissue paper and it came back and hit you.' The plum pudding model couldn't explain these observations.
Worked example: Analysing the Scattering Results
Explain what each observation from the alpha scattering experiment tells us about atomic structure.
About 99% of alpha particles showed no deflection — This means atoms are mostly empty space - there's nothing to deflect most particles
A small percentage scattered at angles > 90° — These particles came close to something with strong positive charge that repelled them
About 1 in 8000 particles reflected back towards source — These hit something very small, very dense, and very positively charged head-on
Positive charge and mass must be concentrated in tiny central nucleus — Only this arrangement explains all three observations together
Answer: The atom has a tiny, dense, positively charged nucleus at its centre, surrounded by mostly empty space where electrons orbit.
The key insight was that if positive charge was spread throughout the atom (as in plum pudding), no alpha particle should bounce straight back. The only way to get such dramatic deflections was if all the positive charge was concentrated in a tiny central region.
5. How Scientific Models Change
Scientific models aren't permanent truths - they're tools that help us understand the world. When new evidence contradicts a model's predictions, scientists must modify or replace the model entirely.
The atomic model shows this process perfectly. Thomson's plum pudding model explained the evidence available in 1904. But when Rutherford's experiment provided new evidence, the model had to change. This is how science progresses - through evidence, not opinion.
6. Exam Technique and Common Mistakes
Exam questions on atomic models often ask you to compare the two models or explain why evidence led to change. The key is being specific about what the evidence showed and why it contradicted the old model.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
What was the alpha particle scattering experiment?
The alpha particle scattering experiment was Rutherford's experiment in which alpha particles were fired at thin gold foil to investigate atomic structure by observing how the particles were deflected. Its results led to the nuclear model replacing the plum pudding model.
What are alpha particles?
Alpha particles are helium nuclei with a +2 charge. Rutherford fired them at extremely thin gold foil in the alpha particle scattering experiment to investigate atomic structure.
What is the nucleus of an atom?
The nucleus is the tiny, dense, positively charged centre of the atom, with almost all its mass and all its positive charge. In the nuclear model it is surrounded by mostly empty space, where the electrons orbit.
Why do scientific models change?
Scientific models change when new evidence contradicts their predictions, so scientists must modify or replace them. Thomson's plum pudding model explained the evidence available in 1904, but Rutherford's experiment provided new evidence, so the model had to change.
Who proposed the nuclear model of the atom?
Ernest Rutherford proposed the nuclear model of the atom in 1911, after his alpha particle scattering experiment. In his model, almost all the mass and positive charge is in a tiny central nucleus, with electrons orbiting around it.
What did the alpha scattering experiment show?
Most alpha particles passed straight through, showing atoms are mostly empty space. Some were deflected at large angles and about 1 in 8000 bounced straight back, showing the positive charge and mass are concentrated in a tiny, dense central nucleus.
What does it show that some alpha particles bounced straight back?
Some alpha particles bounced back because they hit something very small, very dense and very positively charged head-on. About 1 in 8000 reflected back towards the source, showing the positive charge and mass are concentrated in a tiny nucleus.
Why did most alpha particles pass straight through the gold foil?
Most alpha particles, about 99%, passed straight through the gold foil because atoms are mostly empty space, so there was nothing to deflect them. Only a few came close to the nucleus.
How many alpha particles bounced back in Rutherford's experiment?
About 1 in 8000 alpha particles bounced straight back towards the source in Rutherford's experiment. Most passed straight through and some were deflected at large angles, more than 90°.
What did the plum pudding model predict for alpha scattering?
The plum pudding model predicted that alpha particles would pass through the gold foil with only slight deflections, like bullets through jelly. With positive charge spread throughout the atom, no alpha particle should bounce straight back.
When was the plum pudding model proposed?
Thomson proposed the plum pudding model in 1904, after discovering electrons. It pictured the atom as a sphere of positive charge with negative electrons embedded throughout, and it was replaced by Rutherford's nuclear model in 1911.
What did Rutherford say about the alpha scattering results?
Rutherford famously compared the alpha scattering results to firing a 15-inch shell at tissue paper and having it come straight back. The plum pudding model could not explain particles bouncing straight back.
Why were some alpha particles deflected at large angles?
Some alpha particles were deflected at large angles, more than 90°, because they came close to something with a strong positive charge that repelled them. Only a concentrated positive nucleus could cause such dramatic deflections.