Neptunium and plutonium.
Fermi had tried to form element number 93 by bombarding uranium with neutrons (see 1934). Hahn and Meitner had shown that the result was nuclear fission (see 1939). The two were not mutually exclusive, however. It was possible that some uranium nuclei might undergo fission and others might undergo the kind of changes that would produce element number 93.
In 1940 the American physicists Edwin Mattison McMillan (b. 1907) and Philip Hauge Abelson (b. 1913), studying uranium that had been bombarded with neutrons, detected a beta particle with a half-life of 2.3 days. When they had tracked this down, they announced on June 8, 1940, that they had located traces of element number 93.
Since uranium had been named for the planet Uranus (see 1789), the new element, lying beyond uranium, was named neptunium, for Neptune, the planet lying beyond Uranus.
Since the neptunium isotope that had been located emitted beta particles, it had to gain another unit in atomic number and produce element number 94, which was named plutonium, for the planet Pluto. In this part of the work, the American physicist Glenn Theodore Seaborg (b. 1912) was prominent.
Neptunium and plutonium were the first of the transuranium elements to be discovered. There would be others.
Seaborg recognized that the transuranium elements were part of a series analogous to the rare earth elements. There were fifteen elements running from lanthanum (number 57) to lutetium (number 71) inclusive (with number 61 still undiscovered), and they were now called the lanthanides after the first member. The second group would be the fifteen elements running from actinium (number 89) to element number 103 inclusive, and they were named the actinides. Six of the actinides were now known, and nine remained to be discovered.
For their work on transuranium elements, McMillan and Seaborg were awarded the Nobel Prize for chemistry in 1951.
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|Publication:||Asimov's Chronology of Science & Discovery, Updated ed.|
|Article Type:||Reference Source|
|Date:||Jan 1, 1994|
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