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Democritus developed the idea of atoms by dividing up materials and reducing them to smaller and smaller particles and he called them atoma.
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John Dalton suggested that all matter was made up of tiny spheres that could move around with perfect elasticity and he called them atoms.
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Joseph John Thompson discovered that atoms could sometimes eject a smaller negative particle which he called an electron.
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Thompson developed the idea that an atom was made up of unevenly scattered electrons within an elastic sphere. That is surrounded by a soup of positive charge to balance the electron's charge.
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Ernest Rutherford oversaw Geiger and Marsden carrying out his experiment where they fired Hileum nuclei at gold foil that was only a few atoms thick. They found that most of them passed through but 1 in 10,000 got hit. He concluded that the positive charge was all in a central nucleus, holding the electrons in place.
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Niels Bohr refined Rutherford's idea by adding that the electrons were in orbits and each orbit could only conatin a set of electrons.