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Theory: He formulated an atomic theory of matter with 4 postulates. Matter is composed of atoms separated by empty space through which atoms move.
When: Fifth Century B.C.
Where: Greece

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Theory: Proposed that all matter was composed of atoms.
When: 1808
Where: UK

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Theory: Realized that the Physical and chemical properties of elements were related their atomic mass in a periodic way and arranged groups of elements with similar properties.
When: 1869
Where: Russia

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Theory: He demonstrated that cathode ray tubes were negatively charged. He realized that the model of an atom accounted for negative or Positive particles.
When: 1897
Where: UK

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Theory: Demonstrated that the atom has tiny and heavy nucleus
When: 1911
Where: UK

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Theory: Said that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted.
When: 1913
Where: Denmark

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He described the likelihood of finding an electron in a certain position. This atomic is known as the quantum mechanical model of the atom.
When: 1926
Where: Austria