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John Dalton proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together.
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Dmitri Mendeleev created the periodic table.
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J.J. Thompson 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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Niels Bohr proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.
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Erwin Schroedinger developed wave mechanics which describe the behaviour of quantum systems for bosons.