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The Thomson Model and Thomson Scattering

Saturday, February 5, 2022

The Thomson Model (or Plum Pudding Model) of the Atom

For an atom with ZZ electrons, there is a sphere of ZeZe positive charge encapsulating those electrons.

Scattering experiments

Scattering experiments are when atoms are bombarded by charged particles to observe the deflection they experience.

In scattering experiments, the particles are deflected by the electric charges within the atom (the positive charge in the atom repels a positively charged particle while the electrons attract it). The positive charges have a much greater effect than the electrons, since the mass of the electrons is so much less than the particle.

Since experiments on a single atom are nearly impossible, scattering experiments are typically done to metal foils. The average scattering angle is θav\theta_{av}, and the atomic radius is denoted RR. The total scattering angle, θ\theta, is the angle of deflection the particle experiences through the foil as a whole, can be approximated as such:

θNθav\theta \simeq \sqrt{N}\theta_{av}

The Thomson model is accurate when predicting the average scattering angle for a single atom through a foil about 10410^4 atoms thick, but fails completely for the probability of scattering at large angles.

Rutherford's experiment

Ernest Rutherford conducted an experiment which showed that alpha-particles scattered at angles greater than 90°90\degree with a probability of 10410^{-4}, which is significantly more likely than Thomson's model predicts (his model predicts 10300010^{-3000}). This led Rutherford to propose a model of the atom where the positive charge is not equally distributed but rather concentrated in a very small region.