An element is a substance each of whose atoms contains the same number of electrons. Until recently, scientists thought there were 92 elements, ranging from hydrogen (with one electron circling round its nucleus) to uranium (with 92). All the intervening numbers had been accounted for. So the chemists sat back, feeling that their long search for elements had been completed.
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...
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