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Concept
Geiger–Marsden experiments
Summary
The Geiger–Marsden experiments (also called the Rutherford gold foil experiment) were a landmark series of experiments by which scientists learned that every atom has a nucleus where all of its positive charge and most of its mass is concentrated. They deduced this after measuring how an alpha particle beam is scattered when it strikes a thin metal foil. The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.
The prevailing theory of atomic structure at the time of Rutherford's experiments was the "plum pudding model". This model was devised by J. J. Thomson. Thomson had discovered the electron and believed that every atom was a sphere of positive charge throughout which the electrons were distributed, a bit like raisins in a Christmas pudding. The existence of protons and neutrons was unknown at this time. This model was based entirely on classical (Newtonian) physics; the current accepted model uses quantum mechanics.
Thomson's model was not universally accepted even before Rutherford's experiments. Thomson himself was never able to develop a complete and stable model of his concept. Japanese scientist Hantaro Nagaoka rejected Thomson's model on the grounds that opposing charges cannot penetrate each other. He proposed instead that electrons orbit the positive charge like the rings around Saturn.
An alpha particle is a sub-microscopic, positively charged particle of matter that is spontaneously emitted from certain radioactive elements. Rutherford discovered their existence and deduced that they were essentially helium atoms without electrons, though nobody knew about protons and neutrons at the time.
According to Thomson's model, if an alpha particle were to collide with an atom, it would just fly straight through, its path being deflected by at most a fraction of a degree. At the atomic scale, the concept of "solid matter" is meaningless. The Thomson atom is a sphere of electric charge anchored in space by its mass.
Official source
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