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Democritus, a Greek philosopher, co-originated the thought that all matter is composed of indivisible elements.
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John Dalton developed a theory that matter is simply composed of atoms of different weights and is combined in ratios by weight. Also proposed that these atoms are spherical, and are in motion.
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J.J. Thomson determined the charge to mass ratio of electrons
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Rutherford discovered alpha, beta, and gamma rays in radiation
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In 1913, Neils Bohr, a student of Rutherford's, developed a new model of the atom. He proposed that electrons are arranged in concentric circular orbits around the nucleus.
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Niels Bohr proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.
