Word: neptunium
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Dates: during 1940-1949
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...others: neptunium, No. 93; plutonium, No. 94; americium...
Chemists of the Manhattan Project were having mythology trouble. They had created two new elements, Nos. 95 and 96 (TIME, Nov. 26). But when they tried to name them, they were stumped. Uranium (No. 92), neptunium (No. 93), plutonium (No. 94) had been named after the last three outer planets, Uranus, Neptune and Pluto...
They sat up again with a start in 1940, when University of California scientists produced a new, "synthetic" element (neptunium) by bombarding uranium with neutrons from a cyclotron. Neptunium has 93 electrons, which meant that the list of known elements was growing at the heavy end. It grew some more that same year when Dr. Glenn T. Seaborg and co-workers synthesized plutonium, which has 94 electrons...
...possibility. When natural uranium (one part U-235, 140 parts U-238) is bombarded with slow neutrons, more happens than the cracking of the U-235 particles. Some of the neutrons produced by these fissions are absorbed by the more phlegmatic U-238; This forms a new, unstable element, neptunium, which soon turns into plutonium...