Use alpha scattering evidence for the nuclear atom
Connect straight paths and rare large deflections to a small massive positive nucleus.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recognise alpha charge and electrostatic repulsion.
- Distinguish observations from model inferences.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the large majority passing nearly straight chiefly support?
What you will learn
- Connect straight paths and rare large deflections to a small massive positive nucleus.
- Explain how the full alpha-scattering pattern supports small size, concentrated mass and positive nuclear charge.
Link each observation to a structural inference
A directed alpha beam passing through a very thin metal foil produces mostly nearly straight paths, some deflections and a very small number of large-angle or backward paths. Most particles passing with little deflection supports an atom with mostly empty space and a nucleus occupying a very small region. The beam is directed before scattering; this does not contradict random emission direction at a radioactive source.
Rare large deflections show that a strong interaction is concentrated in a small region. A massive centre can change an alpha particle’s motion substantially without being swept away like a very light electron. This supports the nucleus containing most atomic mass. Positive alpha particles are repelled by the positive nucleus, explaining deflections from close approaches.
Read evidence without overstating it
The evidence supports the small positive massive nucleus model; it does not directly photograph electron orbits or make the atom completely empty. Most undeflected particles and a few backward paths must both be explained. A uniform spread of light positive matter throughout the atom would not account well for the rare strong deflections.
Use a supplied trace diagram or counts from an authorised demonstration. The drawn rays are selected examples, not the true relative frequency of each category, and the enlarged nucleus is not to scale. Do not estimate nuclear radius by measuring this schematic or treat a few plotted paths as statistical percentages.
Sources for this lesson
Pause and explain
Why do rare strong alpha deflections support a massive nucleus rather than light electrons?
Worked example
A foil record shows most alpha particles almost straight and a few turning through large angles. What three nuclear-model features does this support?
Show the worked solution
- Most pass through regions outside a tiny centre.
- Strong rare deflections require concentrated massive positive matter.
- Positive alpha particles are repelled by that positive centre.
Answer A very small positive nucleus containing most atomic mass, with mostly empty surrounding space.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
Explain the straight-path majority in thin-foil alpha scattering.
Give me a hint
Connect a common observation to the size of the interacting region.
Compare my reasoning
- Most particles pass with little deflection.
- Most routes miss the tiny nucleus.
- The atom has a large mostly empty surrounding region.
A small nucleus and mostly empty atomic space.
Look for these in your work
- State observation and inference.
- Do not call the atom entirely empty.
Explain why some positive alpha particles turn back from a close nuclear approach.
Give me a hint
Use nuclear charge and concentrated mass.
Compare my reasoning
- Both alpha and nucleus are positive.
- A close approach can give strong repulsion.
- The massive centre supports a large change of alpha motion.
Strong repulsion from a small massive positive nucleus.
Look for these in your work
- State the force sign.
- Link concentration and large deflection.
A drawing has two straight and one backward ray. Can it establish one-third of particles return or the nucleus size?
Give me a hint
Separate illustrative selection from measured statistics and scale.
Compare my reasoning
- Only selected paths are drawn.
- Their frequencies are not a sampled count table.
- The enlarged centre is not a measured radius.
Neither frequency nor nuclear radius follows from the schematic alone.
Look for these in your work
- Identify two representation limits.
- Request actual counts or dimensional evidence.
Common mistakes
- Explain all three features.
- Selected rays are not frequency data.
- Scattering evidence does not trace electron orbits.
What can you explain now?
What is the sign of an alpha particle?
Compare with the explanation
Positive
An alpha particle is positively charged, so repulsion from a positive nucleus can explain scattering.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain how the full alpha-scattering pattern supports small size, concentrated mass and positive nuclear charge.
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.