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In 1799, Joseph Proust discovered the Law of Definite Proportion which states that a chemical compound always contains exactly the same proportion of elements by mass.
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In 1800, he developed the atomic theory.
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In 1811, Avogadro's law states that equal volumes of all gases at the same temperature and pressure contain the same number of molecules.
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In 1869, Mendeleev created the first periodic table of elements, and he used it to predict the elements not discovered yet.
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In 1869, Thomson and his two partners, John S. Townsend and H. A. Wilson, performed experiments indicating that cathode rays were unique particles. They discovered the electron.
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In 1898, Marie and her husband Pierre wrote a paper revealing the existences of radioactivity and two elements, polonium and radium.
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In 1909, Hans Geiger was the co-inventor of the Geiger counter and participated in the Geiger-Marsden experiment which discovered the Atomic nucleus.
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In 1913, Niels Bohr discovered that electrons resided in levels with the energy determined by electron's orbits around the nucleus. He believed that the electrons could move between the levels by absorbing photons at specific frequencies.
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In 1914, Rutherford discovered that the structure of an atom has a nucleus of positive charges surrounded by the negatively charged electrons.
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In 1939, Fermi and his partner Leó Szilárd created the first artificial self-sustaining nuclear chain reaction.
