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Democritus made many very important discoveries in the span of his lifetime but the greatest was that of the atom. If it weren't for Democritus, there would be no modern atomic theory and there would still be questions left to answer that were answered hundreds of years ago.
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John Dalton developed the first useful atomic theory of matter around 1803. In the course of his studies on meteorology, Dalton concluded that evaporated water exists in air as an independent gas.
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In 1897 in Cambridge, J J Thomson experimented on cathode rays. In Britain, physicists had argued these rays were particles, but German physicists disagreed, thinking they were a type of electromagnetic radiation.
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1911 His second discovery, the nuclear model of the atom, became the basis for how we see the atom today: a tiny nucleus surrounded by orbiting electrons.
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1912 Bohr joined Rutherford. He realized that Rutherford's model wasn't quite right. By all rules of classical physics, it should be very unstable.
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In 1932 Chadwick proved the existence of neutrons. He used a different method for tracking particle radiation and knocked the protons out of the nucleus. But the nucleus still had some mass, what proved that neutrons exist.
