Fermium

. . Fermium was first discovered in the fallout from the 'Ivy Mike' nuclear test (1 November 1952), the first successful test of a hydrogen bomb. 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 244Pu). 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. 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. ==Isotopes==

There are 20 known isotopes of fermium, with atomic weights of 241 to 260, of which Fm is the longest-lived with a half-life of 100.5 days. Other isotopes are considerably shorter: Fm has a half-life of 3 days, Fm of 25.4 h, Fm of 20.1 h, while Fm of 5.3 h, Fm of 3.2 h, and Fm of 2.67 hours. All the remaining ones have half-lives ranging from 30 minutes to less than a millisecond. The neutron capture product of fermium-257, Fm, undergoes spontaneous fission with a half-life of just 370(14) microseconds; Fm and Fm also undergo spontaneous fission (t1/2 = 1.5(3) s and 4 ms respectively). This means that neutron capture does not create nuclides with a mass number greater than 257, unless carried out at extremely high flux, as in a nuclear explosion (or the astrophysical r-process), since no accessible fermium isotopes undergo beta minus decay to the next element, mendelevium. Because of this impediment in forming heavier isotopes, these short-lived isotopes Fm constitute the "fermium gap." Deliberate attempts at nuclear explosion synthesis, however, also failed to create heavier nuclei, and as a result were deemed not worth continuing, ==Occurrence==

Production : chromatographic separation of Fm(100), Es(99), Cf, Bk, Cm and Am Fermium is produced by the bombardment of lighter actinides with neutrons in a nuclear reactor. Fermium-257 is the heaviest isotope that is obtained via neutron capture, and can only be produced in picogram quantities. The major source is the 85 MW High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory in Tennessee, USA, which is dedicated to the production of transcurium (Z > 96) elements. Lower mass fermium isotopes are available in greater quantities, though these isotopes (254Fm and 255Fm) are comparatively short-lived. In a "typical processing campaign" at Oak Ridge, tens of grams of curium are irradiated to produce decigram quantities of californium, milligram quantities of berkelium and einsteinium, and picogram quantities of fermium. However, nanogram Smaller cations form more stable complexes with the α-hydroxyisobutyrate anion, and so are preferentially eluted from the column. 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). , 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), 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. 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. 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. Natural occurrence Because of the short half-life of all known isotopes of fermium, any primordial fermium, that is fermium present on Earth during its formation, has decayed by now. Synthesis of fermium from naturally occurring uranium and thorium in the Earth's crust requires multiple neutron captures, which is extremely unlikely. Therefore, most fermium is produced on Earth in laboratories, high-power nuclear reactors, or in nuclear tests, and is present for only a few months afterward. The transuranic elements up to fermium should have been present in the natural nuclear fission reactor at Oklo, but any quantities produced then would have long since decayed away. ==Chemistry==

alloy used for measuring the enthalpy of sublimation of fermium metal The chemistry of fermium has only been studied in solution using tracer techniques, and no solid compounds have been prepared. Under normal conditions, fermium exists in solution as the Fm3+ ion, which has a hydration number of 16.9 and an acid dissociation constant of 1.6 (pK = 3.8). Fm forms complexes with a wide variety of organic ligands with hard donor atoms such as oxygen, and these complexes are usually more stable than those of the preceding actinides. It is believed that the bonding in the complexes of the later actinides is mostly ionic in character: the Fm ion is expected to be smaller than the preceding An ions because of the higher effective nuclear charge of fermium, and hence fermium would be expected to form shorter and stronger metal–ligand bonds. 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. 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 polarographic measurements. ==Toxicity==

Though few people come in contact with fermium, the International Commission on Radiological Protection has set annual exposure limits for the two most stable isotopes. For fermium-253, the ingestion limit was set at 10 becquerels (1 Bq equals one decay per second), and the inhalation limit at 10 Bq; for fermium-257, at 10 Bq and 4,000 Bq respectively. ==Notes and references==

Notes References ==Further reading==