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Gold foil experiment. Animation depicting the actual outcome of Rutherford's 1909 experiment to probe the structure of an atom. At left, a source of alpha radiation is firing alpha particles (helium nuclei) at a thin sheet of gold foil (down centre). Most of the alpha particles pass straight through the foil, as was expected, but some deviate by large angles, even bouncing back at the source. The inset shows a close-up of a gold atom, revealing that its positive charge (red) is tightly concentrated in a small, dense nucleus, with the negative electrons (blue) a relatively long way from it. This demonstration led to the downfall of the prevailing 'plum pudding' model of the atom, which postulated that the electrons were studded randomly in a diffuse cloud of positive charge. For a clip showing the result expected were the plum pudding model correct, see K003 3970.
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