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Fermium is a synthetic chemical element; it has Chemical symbol Fm and atomic number 100. It is an actinide and the heaviest element that can be formed by neutron bombardment of lighter elements, and hence the last element that can be prepared in macroscopic quantities, although pure fermium metal has not been prepared yet. A total of 20 isotopes are known, with 257Fm being the longest-lived with a half-life of 100.5 days.
Fermium was discovered in the debris of the Ivy Mike hydrogen bomb explosion in 1952, and named after Enrico Fermi, one of the pioneers of nuclear physics. Its chemistry is typical for the late actinides, with a preponderance of the +3 oxidation state but also an accessible +2 oxidation state. Owing to the small amounts of produced fermium and all of its isotopes having relatively short half-lives, there are currently no uses for it outside basic scientific research.
Element 99 (einsteinium) was quickly discovered on filter papers which had been flown through clouds from the explosion (the same sampling technique that had been used to discover ). It was then identified in December 1952 by Albert Ghiorso and co-workers at the University of California at Berkeley. They discovered the isotope 253Es (half-life ) that was made by the neutron capture of 15 by uranium-238 nuclei – which then underwent seven successive :
Some 238U atoms, however, could capture another amount of neutrons (most likely, 16 or 17).
The discovery of fermium () required more material, as the yield was expected to be at least an order of magnitude lower than that of element 99, and so contaminated coral from the Enewetak atoll (where the test had taken place) was shipped to the University of California Radiation Laboratory in Berkeley, California, for processing and analysis. About two months after the test, a new component was isolated emitting high-energy α-particles () with a half-life of about a day. With such a short half-life, it could only arise from the β− decay of an isotope of einsteinium, and so had to be an isotope of the new element 100: it was quickly identified as 255Fm ().
The discovery of the new elements, and the new data on neutron capture, was initially kept secret on the orders of the U.S. military until 1955 due to Cold War tensions.Fields, P. R.; Studier, M. H.; Diamond, H.; Mech, J. F.; Inghram, M. G. Pyle, G. L.; Stevens, C. M.; Fried, S.; Manning, W. M. (Argonne National Laboratory, Lemont, Illinois); Ghiorso, A.; Thompson, S. G.; Higgins, G. H.; Seaborg, G. T. (University of California, Berkeley, California): "Transplutonium Elements in Thermonuclear Test Debris", in: Nevertheless, the Berkeley team was able to prepare elements 99 and 100 by civilian means, through the neutron bombardment of plutonium-239, and published this work in 1954 with the disclaimer that it was not the first studies that had been carried out on the elements. The "Ivy Mike" studies were declassified and published in 1955.
The Berkeley team had been worried that another group might discover lighter isotopes of element 100 through ion-bombardment techniques before they could publish their classified research, and this proved to be the case. A group at the Nobel Institute for Physics in Stockholm independently discovered the element, producing an isotope later confirmed to be 250Fm () by bombarding a target with oxygen-16 ions, and published their work in May 1954. Nevertheless, the priority of the Berkeley team was generally recognized, and with it the prerogative to name the new element in honour of Enrico Fermi, the developer of the first artificial self-sustained nuclear reactor. Fermi was still alive when the name was proposed, but had died by the time it became official.
After production, the fermium must be separated from other actinides and from lanthanide fission products. This is usually achieved by ion-exchange chromatography, with the standard process using a cation exchanger such as Dowex 50 or TEVA eluted with a solution of ammonium α-hydroxyisobutyrate. Smaller cations form more stable complexes with the α-hydroxyisobutyrate anion, and so are preferentially eluted from the column. A rapid fractional crystallization method has also been described.
Although the most stable isotope of fermium is 257Fm, with a half-life of 100.5 days, most studies are conducted on 255Fm ( t1/2 = 20.07(7) hours), since this isotope can be easily isolated as required as the decay product of 255Es ( t1/2 = 39.8(12) days).
The atmospheric results were supplemented by the underground test data accumulated in the 1960s at the Nevada Test Site, as it was hoped that powerful explosions conducted in confined space might result in improved yields and heavier isotopes. Apart from traditional uranium charges, combinations of uranium with americium and thorium have been tried, as well as a mixed plutonium-neptunium charge. They were less successful in terms of yield (of material), which was attributed to stronger losses of heavy isotopes due to enhanced fission rates in heavy-element charges. Isolation of the products was found to be rather problematic, as the explosions were spreading debris through melting and vaporizing rocks under the great depth of 300–600 meters, and drilling to such depth in order to extract the products was both slow and inefficient in terms of collected volumes.Seaborg, p. 40
Among the nine underground tests, which were carried between 1962 and 1969 and codenamed Anacostia (5.2 TNT equivalent, 1962), Kennebec (<5 kilotons, 1963), Par (38 kilotons, 1964), Barbel (<20 kilotons, 1964), Tweed (<20 kilotons, 1965), Cyclamen (13 kilotons, 1966), Kankakee (20-200 kilotons, 1966), Vulcan (25 kilotons, 1966) and Hutch (20-200 kilotons, 1969), United States Nuclear Tests July 1945 through September 1992 , DOE/NV--209-REV 15, December 2000 the last one was most powerful and had the highest yield of transuranium elements. In the dependence on the atomic mass number, the yield showed a saw-tooth behavior with the lower values for odd isotopes, due to their higher fission rates. The major practical problem of the entire proposal, however, was collecting the radioactive debris dispersed by the powerful blast. Aircraft filters adsorbed only about 4 of the total amount and collection of tons of corals at Enewetak Atoll increased this fraction by only two orders of magnitude. Extraction of about 500 kilograms of underground rocks 60 days after the Hutch explosion recovered only about 10 of the total charge. The amount of transuranium elements in this 500-kg batch was only 30 times higher than in a 0.4 kg rock picked up 7 days after the test. This observation demonstrated the highly nonlinear dependence of the transuranium elements yield on the amount of retrieved radioactive rock.Seaborg, p. 43 In order to accelerate sample collection after the explosion, shafts were drilled at the site not after but before the test, so that the explosion would expel radioactive material from the epicenter, through the shafts, to collecting volumes near the surface. This method was tried in the Anacostia and Kennebec tests and instantly provided hundreds of kilograms of material, but with actinide concentrations 3 times lower than in samples obtained after drilling; whereas such a method could have been efficient in scientific studies of short-lived isotopes, it could not improve the overall collection efficiency of the produced actinides.Seaborg, p. 44
Though no new elements (apart from einsteinium and fermium) could be detected in the nuclear test debris, and the total yields of transuranium elements were disappointingly low, these tests did provide significantly higher amounts of rare heavy isotopes than previously available in laboratories. For example, 6 atoms of Fm could be recovered after the Hutch detonation. They were then used in the studies of thermal-neutron induced fission of Fm and in discovery of a new fermium isotope Fm. Also, the rare isotope Cm was synthesized in large quantities, which is very difficult to produce in nuclear reactors from its progenitor Cm; the half-life of Cm (64 minutes) is much too short for months-long reactor irradiations, but is very "long" on the explosion timescale.Seaborg, p. 47
Fermium(III) can be fairly easily reduced to fermium(II), for example with samarium(II) chloride, with which fermium(II) coprecipitates. In the precipitate, the compound fermium(II) chloride (FmCl) was produced, though it was not purified or studied in isolation. (1992). 9780412301209, Chapman & Hall. ISBN 9780412301209 The electrode potential has been estimated to be similar to that of the ytterbium(III)/(II) couple, or about −1.15 V with respect to the standard hydrogen electrode, a value which agrees with theoretical calculations. The Fm/Fm couple has an electrode potential of −2.37(10) V based on Polarography measurements.
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