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Atoms were thought to be the smallest possible division of matter until 1897 when J.J. Thomson discovered the electron
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Thomson's plum pudding model was disproved in 1909
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In the gold foil experiment, Hans Geiger and Ernest Marsden shot alpha particles at a thin sheet of gold, measuring their deflection with a fluorescent screen. a small fraction of the alpha particles experienced heavy deflection.This led Rutherford to propose a planetary model in which a cloud of electrons surrounded a small, compact nucleus of positive charge.
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In 1913 radiochemist Frederick Soddy discovered that there seemed to be more than one element at each position on the periodic table. Then the isotope was discovered
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Rutherford concluded that the hydrogen nuclei emerged from the nuclei of the nitrogen atoms themselves.