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electrons could only occupy particular orbits determined by the quantum of action and that electromagnetic radiation from an atom occurred only when an electron jumped to a lower-energy orbit.
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started building up the periodic table by adding electrons one after another to the atom according to his atomic model.
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Started work on a cyclotron after realizing theoretical physics went from studying an atom to its nucleus
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When Hitler came to power, used his connections to get physicists out Germany and into America to continue work