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In 1869 Russian chemist Dimitri Mendeleev started the development of the periodic table, arranging chemical elements by atomic mass. He predicted the discovery of other elements, and left spaces open in his periodic table for them. In 1897 English physicist J. J. Thomson first discovered electrons; small negatively charged particles in an atom. John Townsend and Robert Millikan determined their exact charge and mass.
In 1900 Bequerel discovered that electrons and beta particles as identified by the Curies are the same thing.
In 1903 Rutherford announced that radioactivity is caused by the breakdown of atoms.
In 1911 Rutherford and German physicist Hans Geiger discovered that electrons orbit the nucleus of an atom.
In 1913 Bohr discovered that electrons move around a nucleus in discrete energy called orbitals. Radiation is emitted during movement from one orbital to another.
In 1914 Rutherford first identified protons in the atomic nucleus. He also transmutated a nitrogen atom into an oxygen atom for the first time. English physicist Henry Moseley provided atomic numbers, based on the number of electrons in an atom, rather than based on atomic mass.
In 1932 James Chadwick first discovered neutrons, and isotopes were identified. This was the complete basis for the periodic table. In that same year Englishman Cockroft and the Irishman Walton first split an atom by bombarding lithium in a particle accelerator, changing it to two helium nuclei.
In 1945 Glenn Seaborg identified lanthanides and actinides (atomic number >92), which are usually placed below the periodic table.
Sources
Manhattan Project
Wikipedia
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