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Democritus was an important figure in history because he created the idea of an atom and lead to further discovery.
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John Dalton proposed that all matter is composed of atoms. Also all atoms of a given element are identical in mass and properties
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In 1897, J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube.
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Rutherford discovered that the atomy has a tiny and heavy nucleus
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Neils Bohr came up with the concept of electrons.
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In 1932 James Chadwick discovered that neutrons have no specific charge and can be found at the core of an atom.