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He discovered the atom in 400 B.C. Democritus suggested that all matter was formed of different types of tiny discrete particles and that the properties of these particles also determined the properties of matter.
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Dalton developed the first useful atomic theory of matter around 1803. The main points of Dalton's atomic theory were: Elements are made of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties.
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He discovered the electron in a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube.
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Millikan measured the charge on an electron in 1909. Milliken worked on an oil-drop experiment in which he measured the charge on a single electron.
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In 1911, Rutherford discovered the nuclear model of the atom, the nucleus. Rutherford was responsible for a remarkable series of discoveries in the fields of radioactivity and nuclear physics.
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Bohr developed the Bohr model of the atom, in which he proposed that energy levels of electrons are discrete and that the electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level (or orbit) to another.
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He discovered the neutron. The discovery of the neutron quickly changed scientists' view of the atom, and Chadwick was awarded the Nobel Prize.
