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7.1.2Nuclear Physics

Alpha Particle Scattering

5 min read 4 parts Lesson 3 of 19 Unit quiz

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Section 1 of 4The Experiment

7.1.2 Alpha Particle Scattering
1
A narrow beam of alpha particles, which are positively charged helium nuclei, is directed at a very thin sheet of gold foil.
2
A detector placed around the foil, such as a fluorescent screen, records where the alpha particles arrive after passing through.
3
Most alpha particles pass straight through the foil with little or no deflection.
4
A few particles are deflected through large angles, and a very small number are reflected almost straight back.

Section 2 of 4Interpreting the Results

Mostly empty space
  • Because most alpha particles pass through unaffected, most of the atom must be empty space.
Small positive nucleus
  • Large deflections and rebounds show a small, dense, positively charged nucleus that repels the alpha particles.
Nucleus position
  • The nucleus contains nearly all the mass of the atom and is located at its centre.
Electrons
  • Electrons are far too light to deflect alpha particles significantly, so their effect is negligible in this experiment.

Section 3 of 4Changing the Model

Changing the Model
Plum pudding model
  • The earlier model, with electrons embedded in a uniform positive sphere, was disproved by the results.
Nuclear model
  • Rutherford's model has a small central nucleus, with electrons moving around it at a distance.
Alpha particles
  • An alpha particle has two protons and two neutrons, giving a relative charge of +2 and a relative mass of 4.
Later refinement
  • The nuclear model was later developed into the modern model in which electrons occupy shells.

Section 4 of 4Exam Checklist & Tips

7.1.2 – Key Points for the Exam

  • Alpha particle A positively charged particle made of two protons and two neutrons.
  • Gold foil A very thin sheet of gold used in Rutherford's scattering experiment.
  • Nucleus The small, dense, positively charged centre of an atom.
  • Scattering The deflection of particles from their original path by a force or collision.
  • Plum pudding model An earlier model of the atom in which electrons were embedded in a positive sphere.

Common Mistakes

  • Saying the alpha particles were absorbed by the gold; most of them passed straight through.
  • Concluding that the atom is a solid sphere, which the results disprove.
  • Stating that the electrons caused the large deflections; the deflections are due to the nucleus.
  • Forgetting that alpha particles are positively charged, so they are repelled by the nucleus.
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