Rutherford Alpha Particle Scattering Experiment

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Rutherford Alpha Particle Scattering Experiment

Rutherford's alpha particle scattering experiment changed the way we think of atoms.

Before the experiment the best model of the atom was known as the Thomson or "plum pudding" model. The atom was believed to consist of a positive material "pudding" with negative "plums" distributed throughout.

Thomson or plum pudding model

Rutherford directed beams of alpha particles (which are the nuclei of helium atoms and hence positively charged) at thin gold foil to test this model and noted how the alpha particles scattered from the foil.

Rutherford Alpha Particle Scattering Experiment

Rutherford made 3 observations:

  • Most of the fast, highly charged alpha particles went whizzing straight through undeflected. This was the expected result for all of the particles if the plum pudding model was correct.
  • Some of the alpha particles were deflected back through large angles. This was not expected.
  • A very small number of alpha particles were deflected backwards! This was definitely not as expected. Rutherford later remarked "It was as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back at you!"

To explain these results a new model of the atom was needed.

orbiting negative electron

In this model the positive material is concentrated in a small but massive (lot of mass - not size) region called the nucleus. The negative particles (electrons) must be around the outside preventing one atom from trespassing on its neighbours space to complete this model. The diagram below will help you to understand the results of the experiment.

Try the experiment for yourself and note how the results can be explained using this new model:

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