Rutherford Scattering

The New Zealand-born physicist Ernest Rutherford carried out a famous experiment on the structure of the atom in 1911. He bombarded a thin gold foil with alpha rays (helium-4 nuclei of high velocity, as they occur in many radioactive materials) and analysed the distribution of the deflection angles (scattering angles). This angle describes the changed motion direction of an alpha particle passing an atomic nucleus of the foil.

This app simulates the motion of an alpha particle under the influence of the repulsive electrostatic force (Coulomb force) exerted by a single atomic nucleus of the gold foil. The trajectory of the particle is (approximately) a hyperbola, with the target nucleus as one of the focal points of this hyperbola.

The control panel on the right side has two buttons, one to clear the particle trajectories, and one to "start" the motion of a single alpha particle. For the scattering nucleus, the atomic number (number of protons) can be entered. For the alpha particle, you can vary the velocity and the impact parameter (distance between the scattering nucleus and the asymptotes of the hyperbola). Inputs outside the permitted range are automatically changed. Below the input fields, you can read the values of the deflection angle and the minimal distance. The femtometer unit (fm) is used for this purpose: 1 fm = 10−15 m. Two option fields are used to specify whether asymptotes, impact parameter, and deflection angle should be displayed apart from the particles and orbits involved.

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During the evaluation of the experiment, the measured distribution differed significantly from the calculated distribution for small deflection angles (i.e. large values of the impact parameter), as expected. These deviations can be explained by the influence of the electrons.

For large deflection angles (small impact parameters), the measured distribution matched the precalculated distribution very well. This fact proves that the positive charge inside the atom (i.e. the atomic nucleus) has a very small extension. In other words: Atoms are mostly "empty".