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He theorized that all material bodies are made up of indivisibly small “atoms.”

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The first complete attempt to describe all matter in terms of atoms and their properties.

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experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup."

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He discovered that there are two types of radiation, alpha and beta particles, coming from uranium. He found that the atom consists mostly of empty space, with its mass concentrated in a central positively charged nucleus.

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Niels Bohr proposed that some physical quantities only take discrete values. Electrons move around a nucleus, but only in prescribed orbits, and if electrons jump to a lower-energy orbit, the difference is sent out as radiation.

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 formulated a wave equation that accurately calculated the energy levels of electrons in atoms.