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400 BCE
Democritus proposed the existence of atoms. Limited technology at the time meant this was just an idea and could not be tested.
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Dalton developed his theory of atoms and matter. Experiments in chemistry and careful measurement of results allowed Dalton to support his hypotheses with evidence.
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JJ Thomson discovered the electron. Experiment using electric charges in cathode ray tubes.
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Rutherford developed the planetary model of the atom. Experiment using alpha particles fired at thin gold foil and an early prototype Geiger counter to measure results.
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Bohr developed a model of the atom. Bohr used mathematical models and laws of quantum physics to propose his updated model.
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The neutron was discovered by James Chadwick. Experiments using kinematics and accurate measurements lead to proving the existence of a previously undetected subatomic particle, the neutron.