In 1909, the prevailing theory of atomic structure was the plum pudding model. In this model, proposed by J. J. Thomson, the electrons were thought to be floating around in a soup of positive charge. To help prove or disprove this theory, Ernest Rutherford proposed an experiment.
Rutherford and his associates, Hans Geiger and Ernest Marsden, performed the experiment at the University of Manchester in 1909. They decided to try firing a series of alpha particles (a collection of protons and neutrons) at a very thin sheet of gold foil. The alpha particles were created using a tube of radium bromide gas and the experimenters used a microscope to watch for flashes of light produced by alpha particles striking a zinc sulfide screen.
They thought that the alpha particles would all deflect from the foil, but quickly discovered that most of the alpha particles passed through, with only a minority deflecting. They estimated that about 1 in every 8000 alpha particles were deflected by more than 90 degrees from a foil that was only 6E-8 meters thick (about 200 atoms).
The results from the experiment totally astounded Rutherford, who said, "It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you". This experiment strongly suggested that all of the positive charge in an atom was packed into a very small area, disproving the plum pudding model. In 1911, Rutherford proposed that atoms consisted of protons and neutrons in a small dense nucleus, surrounded by a cloud of orbiting electrons. This model was later revised by Niels Bohr to reflect quantum theory.