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the idea of an atom was thought of by Demorcritus and Leucippus. the atom being an indvisable particle that all matter is made of.
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Robert Boyle published "The Sceptical Chemist" which was a treatise on the distinction between chemistry and alchemy. It also contained some of the earliest ideas of atoms, molecules, and chemical reaction marking the beginning of the history of modern chemistry
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Jakob Berzelius developed a table of atomic weights & introduced letters to symbolize elements. Johann Dobereiner developed groups of 3 elements with similar properties
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Lothar Meyer develops an early version of the periodic table, with 28 elements organized by valence. also Dmitri Mendeleev produced a table based on atomic weights but arranged 'periodically' with elements with similar properties under each other. His Periodic Table included the 66 known elements organized by atomic weights.
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William Ramsay discovered the Noble Gases
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Henry Moseley determined the atomic number of each of the elements and modified the 'Periodic Law'.
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Edwin McMillan and Philip H. Abelson identify neptunium, the lightest and first synthesized transuranium element, found in the products of uranium fission.
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Glenn Seaborg combined transuranic elements.