Infer the nuclear atom from alpha scattering
Use alpha-particle scattering evidence to infer a small, concentrated nucleus in a mostly empty atom.
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 11. These original particle models and equations are study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use atomic number, nucleon number, electric charge and conservation with signed values.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to most alpha particles in the foil experiment?
What you will learn
- Infer nuclear existence and small size from alpha scattering
- Describe the simple nuclear atom with protons, neutrons and orbital electrons
Most alpha particles pass through with little deflection
In the alpha-scattering experiment, a narrow beam of positively charged alpha particles is directed at a very thin metal foil. Most particles pass through nearly undeflected, so most of the atom's volume is empty space in this model.
A small fraction are deflected through large angles, and a very small fraction scatter backwards. A concentrated, positively charged region can strongly repel an alpha particle only when its path passes close to it.
The evidence implies a compact nucleus
The rare large deflections imply that most atomic mass and positive charge are concentrated in a tiny central nucleus. The nucleus is small compared with the atom; it is not a uniform positive sphere filling the atom.
The simple nuclear atom contains protons and neutrons in the nucleus and electrons orbiting around it. This is a model for the syllabus; it is not a scale drawing of atomic structure.
Pause and explain
What explains the rare large-angle alpha deflections?
Worked example
What do the observations that most alpha particles pass straight through but a few turn through large angles imply?
Show the worked solution
- Most paths encounter no strong compact obstacle, so most atomic volume is empty.
- Large deflections require a strong repulsive interaction with concentrated positive charge.
- Therefore the positive charge and most mass occupy a small nucleus.
Answer The atom is mostly empty space, with a small, dense, positively charged nucleus containing most of its mass.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A scattering detector records 99% of alpha particles within a small forward angle and 1% at large angles. State two inferences.
Give me a hint
Relate the common and rare outcomes to atomic volume and concentrated charge.
Compare my reasoning
- The common small deflections imply most atomic volume is empty.
- The rare large deflections imply strong repulsion near a concentrated positive region.
- Infer a small nucleus containing most positive charge and mass.
Most of the atom is empty space; a small positively charged nucleus contains most mass and causes rare large deflections.
Look for these in your work
- Common outcome interpreted
- Rare event interpreted
- Nuclear model inferred
Describe the particles in the simple nuclear atom and state where each is found.
Give me a hint
Separate the nucleus from the region occupied by orbital electrons.
Compare my reasoning
- The nucleus contains protons and neutrons.
- Electrons orbit around the nucleus in the simple model.
- Most of atomic volume is outside the tiny nucleus.
Protons and neutrons are in the small nucleus; electrons orbit around it, with most atomic volume empty.
Look for these in your work
- Three particle types named
- Locations distinguished
- Relative size stated
Why did the few backward-scattered alpha particles challenge a model with positive charge spread uniformly throughout the atom?
Give me a hint
Compare the strength and localisation needed to reverse a fast positive particle.
Compare my reasoning
- A backward turn requires a strong force during a close encounter.
- A diffuse charge distribution would not concentrate enough repulsive interaction into a tiny region.
- The result supports a compact, positively charged nucleus.
The rare reversals require a highly concentrated positive charge and mass, supporting a tiny nucleus rather than a diffuse positive atom.
Look for these in your work
- Strong interaction requirement identified
- Diffuse model contrasted
- Evidence-linked conclusion
Common mistakes
- Saying all alpha particles are deflected by the same angle
- Inferring that the nucleus fills most atomic volume
- Confusing the model of orbital electrons with a literal scale diagram
What can you explain now?
What happens to most alpha particles in the foil experiment?
Compare with the explanation
They pass through with little deflection
This indicates most of the atom is empty space.
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Make it stick
Why do the rare particles matter even though most alpha particles pass straight through?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.