Editing Edwin McMillan (section)

==Discovery of neptunium==
Following the discovery of [[nuclear fission]] in [[uranium]] by [[Otto Hahn]] and [[Fritz Strassmann]] in 1939, McMillan began experimenting with uranium. He bombarded it with [[neutron]]s produced in the Radiation Laboratory's {{convert|37|in|cm|adj=on}} cyclotron through bombarding [[beryllium]] with deuterons. In addition to the [[nuclear fission product]]s reported by Hahn and Strassmann, they detected two unusual radioactive isotopes, one with a half-life of about 2.3 days, and the other with one of around 23 minutes. McMillan identified the short-lived isotope as [[uranium-239]], which had been reported by Hahn and Strassmann. McMillan suspected that the other was an isotope of a new, undiscovered element, with an [[atomic number]] of 93.{{sfn|Jackson|Panofsky|1996|pp=221–222}}

At the time it was believed that element 93 would have similar chemistry to [[rhenium]], so he began working with [[Emilio Segrè]], an expert on that element from his discovery of its [[Homologous series|homolog]] [[technetium]]. Both scientists began their work using the prevailing theory, but Segrè rapidly determined that McMillan's sample was not at all similar to rhenium. Instead, when he reacted it with [[hydrogen fluoride]] (HF) with a strong [[oxidizing agent]] present, it behaved like members of the [[rare-earth element]]s.{{sfn|Jackson|Panofsky|1996|pp=221–223}} Since these comprise a large percentage of fission products, Segrè and McMillan decided that the half-life must have been simply another fission product, titling the article "An Unsuccessful Search for Transuranium Elements".<ref>{{cite journal| last1=Segrè |first1=Emilio |author-link=Emilio Segrè | title=An Unsuccessful Search for Transuranium Elements| date=1939| pages=1104–5|  journal=Physical Review|volume=55| issue=11|bibcode = 1939PhRv...55.1104S |doi = 10.1103/PhysRev.55.1104 }}</ref>

McMillan realized that his 1939 work with Segrè had failed to test the chemical reactions of the radioactive source with sufficient rigor. In a new experiment, McMillan tried subjecting the unknown substance to HF in the presence of a [[reducing agent]], something he had not done before. This reaction resulted in the sample [[Precipitation (chemistry)|precipitating]] with the HF, an action that definitively ruled out the possibility that the unknown substance was a rare earth. In May 1940, [[Philip Abelson]] from the [[Carnegie Institution for Science|Carnegie Institute]] in [[Washington, DC]], who had independently also attempted to separate the isotope with the 2.3-day half-life, visited Berkeley for a short vacation, and they began to collaborate. Abelson observed that the isotope with the 2.3-day half-life did not have chemistry like any known element, but was more similar to uranium than a rare earth. This allowed the source to be isolated and later, in 1945, led to the classification of the [[actinide series]]. As a final step, McMillan and Abelson prepared a much larger sample of bombarded uranium that had a prominent 23-minute half-life from <sup>239</sup>U and demonstrated conclusively that the unknown 2.3-day half-life increased in strength in concert with a decrease in the 23-minute activity through the following reaction:
:<chem>{}^{238}_{92}U + {}^{1}_{0}n -> {}^{239}_{92}U ->[\beta^-] [23\ \ce{min}] \overset{neptunium}{^{239}_{93}Np} ->[\beta^-] [2.355\ \ce{days}] {}^{239}_{94}Pu</chem>

This proved that the unknown radioactive source originated from the decay of uranium and, coupled with the previous observation that the source was different chemically from all known elements, proved beyond all doubt that a new element had been discovered. McMillan and Abelson published their results in an article entitled ''Radioactive Element 93'' in the ''[[Physical Review]]'' on May 27, 1940.{{sfn|Jackson|Panofsky|1996|pp=221–223}}<ref name="EL93">{{cite journal| doi =10.1103/PhysRev.57.1185.2| title =Radioactive Element 93| date =1940| last1=McMillan |first1=Edwin| journal =Physical Review| volume =57| pages =1185–1186| last2 =Abelson| first2 =Philip| issue =12|bibcode = 1940PhRv...57.1185M | doi-access =free}}</ref> They did not propose a name for the element in the article, but they soon decided on "neptunium", since uranium had been named after the planet [[Uranus]], and [[Neptune]] is the next planet beyond in the [[Solar System]].{{sfn|Seaborg|1993|p=289}} McMillan suddenly departed for war-related work at this point, leaving [[Glenn Seaborg]] to pursue this line of research and discover the second transuranium element, [[plutonium]]. In 1951, McMillan shared the [[Nobel Prize in Chemistry]] with Seaborg "for their discoveries in the chemistry of the transuranium elements".<ref>{{cite web | title=The Nobel Prize in Chemistry 1951 | author=Nobel Foundation | url=https://www.nobelprize.org/nobel_prizes/chemistry/laureates/1951/ | access-date=July 16, 2015 | archive-date=July 29, 2015 | archive-url=https://web.archive.org/web/20150729044918/http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1951/ | url-status=live }}</ref>