A Half Century of Artificial Elements
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A Half Century of Artificial Eléments Until 1940, Earth knew only the 91 éléments occurring in nature. This year marks the 50th anniversary of the synthesis and identification of neptuniurri and plutonium, the first artificial éléments beyond atomic number 92 on the periodic table. In the half century since, researchers hâve added 15 more transuranic éléments, up to the as-yet-unnamed élément 109. For centuries alchemists worked at rransmuting one élément into another, most notably in attempts to turn lead into gold or silver. Though they never succeeded, such alchemical experiments eventually laid the groundwork for the science of chemistry and also compiled ah enormous amount of data on chemical reactions and the properties of éléments. Not until 1937 was a truly artificial élément added to the list, when Carlo Perrier and Emilio Segré isolated élément 43 in Italy, filling the mysterious gap between Mo and Ru in the periodic table. They bombarded a molybdenum target with deuterium in a cyclotron; after dissolving the target and adding other elemental tracers, they proved that the bombardment had left an extra proton in some molybdenum nuclei, creating the next élément in the séquence. Scientists at first thought that the élément did occur in nature, but was extremely rare. Only after they were convinced that it was truly artificial did they give it the name of "technetium." Technetium is an anomaly, however, and the rest of the periodic table remained filled up to élément 92, uranium. In 1934 Enrico Fermi had begun bombarding uranium with neutrons, attempting to form the next élément in the periodic table. At first Fermi thought he had been successful, along with other researchers including Otto Hahn, Lise Meitner, and Fritz Strassman, who claimed similar results. However, Ida Noddack wrote a paper in 1934 questioning whether the "new" éléments were actually just fission fragments, unknown radioactive isotopes of lighter éléments. Though Noddack's paper was not noticed at the time, Hahn and Strassman came to the same conclusions in 1939. I n 1940 Edwin M. McMillan and Philip H. Abelson were conducting fission experiments at the University of Califonia, Berkeley, and discovered beta-particle activity with a half-life of 2.3 days. Further investigation led to the announcement, on June 8, 1940, of their discovery of élément 93, produced by 238U in a slow-neutron reaction that did not involve fission. This
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was a rather ironie reversai of Fermi's misleading discovery—McMillan and Abelson had been studying fission and instead created the next élément. Because the élément uranium had been named for the planet Uranus, McMillan and Abelson named their new élément neptunium after the next planet in the solar System, Neptune. That same year, Glenn T. Seaborg, McMillan, Joseph W. Kennedy, and Arthur C. Wahl, also at Berkeley, bombarded uranium oxide with deuterons in a 60-inch cyclotron and succeeded in producing élément 94, 238Pu. Following the planetary scheme, they named the élément plutonium after Pluto, the ninth p
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