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Around 400 B.C.E.
Democritus believed that atoms were uniform, solid, hard, incompressible, and indestructible and that they moved in infinite numbers through empty space until stopped.

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1913
Niels Bohr proposed a theory for the hydrogen atom, based on quantum theory that some physical quantities only take discrete values.

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1932
This atomic model is known as the quantum mechanical model of the atom. Chadwick interpreted this as being composed of particles with a neutral electrical charge and the approximate mass of a proton.

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This british chemist and physicist made the first modern formulation of athomic theory in 1803.

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Thomson was the discover of the electron and then went on to propose a model for the structure of the atom in the year 1897.

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He demonstrated that there were at least two distinct types of radiation: alpha radiation and beta radiation. He discovered that radioactive preparations gave rise to the formation of gases in the year 1899.

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Germany 1927
He contribute in the atomic theory by formulating quantum mechanics and he also discover the the uncertainty principle:“The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa.”

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Austria 1926
Erwin formulate a wave equation that accurately calculated the energy levels of electrons in atoms, which treats electrons as matter waves.