A nickel-clad sphere of neptunium used at Los Alamos National Lab to determine the critical mass of the element
15 March 2024
Neptunium is a transuranic chemical element with atomic number 93 and the symbol Np. It is a hard, silvery, ductile metal. Its practical applications are limited, serving primarily as precursor in plutonium production. It potentially could be used as fuel for nuclear reactors or as the fissionable material in a nuclear weapon but has apparently never been used for these.
There is some dispute over credit for neptunium’s discovery. In 1934, Enrico Fermi claimed to have discovered element 93, but was unable to isolate it chemically. Despite this, he was awarded the Nobel Prize in Physics for this discovery and other related work. Then in 1939 and 1940, Edwin McMillan and Philip Abelson successfully isolated the element and catalogued its chemical properties. McMillan and Abelson are generally credited with the discovery, but some sources give Fermi the credit.
The first public mention of the name neptunium is in an article with a dateline of 8 June 1940 appearing in the Oakland Tribune (that being the local paper for Berkeley, California where McMillan and Abelson conducted their experiments):
Discoverers of the new element are Dr. Edwin M. McMillan, 32, aide to Dr. Ernest O. Lawrence, whose atom-smashing with the cyclotron won him the Nobel price, and Dr. Philip Hauge Abelson, Carnegie Institution, Washington, D.C. The new element, for which the name Neptunium, derived from the planet Neptune, has been suggested by Dr. McMillan, today took its place with atomic types making up the composition of water, air, iron salt and all other matter.
The article also mentions that researchers at Berkeley were close to discovering element 94, which would eventually be dubbed plutonium.
McMillan and Abelson published their findings in Physical Review on 15 June 1940, a week after the Oakland Tribune article appeared, but they did not mention the element’s name in that journal article. As for other coverage of the discovery, the local Tribune got a scoop, but no wire services followed up on it. The 30 August 1941 issue of Science News Letter also notes the discovery and the name, but these two are the only public mentions of the element until after the Hiroshima and Nagasaki bombings in August 1945. The lack of coverage is undoubtedly due to wartime censorship of nuclear research.
There is, however, a classified government report from 19 March 1942, authored by Glenn Seaborg and Arthur Wahl that discusses the naming of elements 93 and 94:
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.
So the names neptunium and plutonium make a connection between the scales of the universe at the very small and the large.
Sources:
“Group of Elements Beyond Uranium Is Found Possible.” Science News Letter, 30 August 1941, 135/1. JSTOR.
McMillan, Edwin and Philip Hauge Abelson. “Radioactive Element 93.” Physical Review, 57, 15 June 1940, 1185–86. Physical Review Journals Archive. DOI: 10.1103/PhysRev.57.1185.2.
Miśkowiec, Pawel. “Name Game: The Naming History of the Chemical Elements—Part 3—Rivalry of Scientists in the Twentieth Century.” Foundations of Chemistry, 12 November 2022. DOI: 10.1007/s10698-022-09452-9.
Oxford English Dictionary, third edition, September 2003, s.v. neptunium, n.
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.]
“U.C. Cyclotron Scientists Find Mysterious New Physical Element” (8 June 1940). Oakland Tribune, 9 June 1940, 4-A/1. NewspaperArchive.com.
Photo credit: Los Alamos National Laboratory, 2002. Wikimedia Commons. Public domain image.