A sphere of plutonium partially surrounded by tungsten-carbide blocks to reflect neutrons back into the sphere; used in 1945 at Los Alamos in experiments to test the critical mass of the element
31 March 2023
Plutonium, element 94, symbol Pu, was first produced in December 1940 at the University of California, Berkeley by a team led by chemist Glenn Seaborg. Plutonium is readily fissionable and along with Uranium-235 is used as the fuel in nuclear reactors and weapons. The element is named for the planet Pluto (now officially defined as a dwarf planet), following the pattern set by uranium and neptunium. Uranium, neptunium, and plutonium have atomic numbers 92, 93, and 94, the same order as the planets Uranus, Neptune, and Pluto.
The initial experiments with and discovery of plutonium were conducted at the University of California, Berkeley, but the discovery was not made public until World War II had ended. Further experimentation and production of plutonium was done under the Manhattan Project. The first recorded use of plutonium is by Seaborg and Arthur Wahl in a then-classified 1942 government report:
Naming the Elements
Since formulae are confusing when the symbols "93” and “94" are used, we have decided to use symbols of the conventional chemical type to designate these elements. Following McMillan, who has suggested the name neptunium (after Neptune, the first planet beyond Uranus) for element 93, we suggest plutonium (after Pluto, the second planet beyond Uranus) for element 94. The corresponding chemical symbols would be Np and Pu.
Public disclosure of the discovery of plutonium came in the wake of the atomic bombings of Hiroshima and Nagasaki. An Associated Press article from 11 August 1945 reads:
The two new elements are neptunium, No. 93, and plutonium, No. 94, which have been added to the previously known 92 varieties of matter. The two are derived from Uranium.
Production of the new elements was disclosed in a scientific review, released by the War Department, of experiments leading up to final production of the atomic bombs.
In 2001, Seaborg would recall how his team came up with the name:
At first we gave the new element no name, simply referring to it as 94. But even that revealed too much for casual conversations around the Faculty Club or the lab, so we adopted the code name of “copper” for element 94 and “silver” for 93. This code worked well enough through 1941, until some experiments required the use of some real copper, which we then referred to as “honest-to-God copper.”
A year after its discovery we finally named our new element. It was so difficult to make, from such rare materials, that we thought it would be the heaviest element ever formed. So we considered names like extremium and ultimium. Fortunately, we were spared the inevitable embarrassment that one courts when proclaiming a discovery to the ultimate in any field by deciding to follow the nomenclatural precedents of the two prior elements.
A new planet had been discovered in 1781 and, like the rest of the planets, named for a Greek or Roman deity—Uranus. A scientist who discovered a heavy new element eight years later named it after the planet: uranium. The planet Neptune was discovered in 1846, so Ed McMillan followed this precedent and named element 93 neptunium. Conveniently for us, the final planet, Pluto, had been discovered in 1930. We briefly considered the form plutium, but plutonium seemed more euphonous. Each element has a one- or two-letter abbreviation. Following the standard rules, this symbol should be Pl, but we chose Pu instead. We thought our little joke might come under criticism, but it was hardly noticed.
In The Making of the Atomic Bomb, Richard Rhodes writes of the symbolism that we “would name element 94 for Pluto, the ninth planet outward from the sun, discovered in 1930 and named for the Greek god of the underworld, a god of earth’s fertility but also the god of the dead.” Any such symbolic meaning, however, was entirely coincidental; I was unfamiliar with the god or why the planet was named for him. We were simply following the planetary precedent.
There is an earlier elemental usage of plutonium, however. Starting in 1816, naturalist Edward David Clarke used plutonium as a name for the element barium, which had already been discovered and named by Humphry Davy. Clarke’s attempt to rename the element never caught on. But one may run across this use of plutonium when engaged in historical research of early nineteenth-century chemistry.
Sources:
Associated Press. “2 Elements Discovered by Atomic Work” (11 August 1945). Atlanta Constitution, 12 August 1945, 11-A. ProQuest Historical Newspapers.
Oxford English Dictionary, third edition, September 2006, s.v. plutonium, n.2.
Seaborg, Glenn T., with Eric Seaborg. Adventures in the Atomic Age. New York: Farrar, Straus and Giroux, 2001, 72. Archive.org.
Seaborg, Glenn T. and Arthur C. Wahl. The Chemical Properties of Elements 94 and 93. US Atomic Energy Commission, AECD-1829, 19 March 1942. HathiTrust Digital Archive. [The version at HathiTrust is a later reprint of the original, published in 1947 or later, when the report was declassified. It contains a note to a 1946 report, so it has been altered from the original in some respects, but the text quoted here would seem to have come unaltered from the original.]
Photo credit: Los Alamos National Laboratory, 1945. Wikimedia Commons. Unless otherwise indicated, this information has been authored by an employee or employees of the Los Alamos National Security, LLC (LANS), operator of the Los Alamos National Laboratory under Contract No. DE-AC52-06NA25396 with the U.S. Department of Energy. The U.S. Government has rights to use, reproduce, and distribute this information. The public may copy and use this information without charge, provided that this Notice and any statement of authorship are reproduced on all copies. Neither the Government nor LANS makes any warranty, express or implied, or assumes any liability or responsibility for the use of this information.