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AS & A-Level Physics 11 — Particles, Nuclei and Radiation

AS & A-Level Physics 11 — Particles, Nuclei and Radiation

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Independent Deckloop revision aligned with the Cambridge International AS & A Level Physics (9702) syllabus, 2025–2027. Not affiliated with or endorsed by Cambridge International Education. Chapter 11 of 26: Particles, nuclei and radiation. Concepts, worked applications and misconception checks.

Physics EN AS & A-Level
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Rutherford's Alpha-Particle Scattering Experiment

The Geiger-Marsden experiment, led by Rutherford, involved firing high-speed alpha particles at a very thin gold foil. The observations were crucial: most alpha particles passed straight through or were deflected by very small angles, indicating that atoms are mostly empty space. A small fraction were deflected at large angles, suggesting a concentrated positive charge. Crucially, a tiny proportion (about 1 in 8000) were deflected by more than 90 degrees, some even backscattering. This led to the inference that atoms possess a very small, dense, positively charged nucleus at their centre, which contains most of the atom's mass and is responsible for the strong electrostatic repulsion observed. The electrons, being much lighter, orbit this nucleus.

Key points

  • Most alpha particles passed through gold foil undeflected, implying atoms are mostly empty space.
  • A small number were deflected at large angles, indicating a concentrated positive charge.
  • A very tiny fraction were backscattered, implying a very dense and small nucleus.
  • The atom consists of a tiny, dense, positively charged nucleus, with electrons orbiting in a vast empty space.

Worked example

Question

A beam of alpha particles is directed at a thin sheet of aluminium foil. Predict the expected observations and explain how they support the nuclear model of the atom.

Solution

1. Observation 1: Most alpha particles would pass straight through the aluminium foil with little or no deflection. This indicates that the aluminium atoms, like gold atoms, are largely empty space.
2. Observation 2: A small number of alpha particles would be deflected through large angles. This suggests the presence of a concentrated positive charge within the aluminium atoms, capable of repelling the positively charged alpha particles.
3. Observation 3: A very tiny fraction of alpha particles would be deflected by more than 90 degrees, some even backscattering. This implies that the positive charge and most of the mass of the aluminium atom are concentrated in an extremely small, dense region, which the alpha particles occasionally hit head-on.

The observations would be similar to those with gold foil: most alpha particles pass through, some are deflected at large angles, and a very few are backscattered. These results collectively support the nuclear model, showing that the aluminium atom, like other atoms, has a tiny, dense, positively charged nucleus surrounded by mostly empty space.

Common pitfalls

  • Assuming that electrons cause significant deflection of alpha particles. Alpha particles are much more massive than electrons, so electron collisions would cause negligible deflection.
  • Believing the nucleus occupies a large proportion of the atom's volume. The experiment clearly showed the nucleus is extremely small compared to the atom.

Prerequisites

  • Basic understanding of atoms as fundamental building blocks.
  • Knowledge of positive and negative charges and electrostatic forces.
Further resources