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Democritus proposed the first theory of the atom around 430 B.C.
He broke a shell into tiny pieces creating a dust. He found the tiniest pieces could not be broke down any smaller, but could fit together by some kind of attraction. -
John Dalton was a British chemist and physicist. He developed a theory ,using gasses, that matter is composed of atoms of different weights and combined in ratios by weight. Dalton also proposed that atoms are spherical and in motion.
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Thompson discovered the electron. He proved one of Dalton's ideas wrong; the atom was not indivisible, but made up of multiple components. He used a cathode-ray tube to study the nature of electric discharge.
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Ernest Rutherford proved Thomson's atomic model to be incorrect. He developed the theory for the structure of the atom and demonstrated the existence of the atomic nucleus using a gold foil experiment.
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Bohr developed an experiment model of atomic structure known as The Bohr Model. He proposed a theory for the hydrogen atom that energy is transferred only in certain well defined quantities. He was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element.
