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Dimitri Mendeleev started the development of the Periodic Table, aranging chemicals by atomic mass.
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Sir William Ramsey discovered nobel gases
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J. J. Thomson first discovered electrons.
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Rutherford announced that radioactivity is caused by the breakdown of atoms
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Rutherford discovered electrons orbit the nucleus of the atom
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Rutherford identified protons in the atomic nucleus
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James Chadwick discovered neutrons, isotopes were indentified
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Glenn Seaborg identified lanthanides and actinides