Fermium (Fm)

Fermium does not occur naturally in the Earth’s crust. It was first identified in December 1952 by American scientists from the Argonne National Laboratory near Chicago, Illinois, the Los Alamos National Laboratory in Los Alamos, New Mexico, and The University of California Laboratory in Berkeley, California in the debris of thermonuclear weapons (Fig. IUPAC.100.1). The element was named for Enrico Fermi, who built the first man-made nuclear reactor. 255Fm (with a half-life of 20 h) was the first fermium isotope identified. Fermium is the heaviest element that can be formed by neutron bombardment of lighter elements and is thus the heaviest element that can be synthesized in macroscopic quantities [632], [633].

Fermium is of interest in particle physics research, but it has no commercial applications. 253Fm was one of the decay products used to confirm synthesis of copernicium in a particle accelerator experiment [634].

Fermium is the heaviest synthetic element that can be formed by neutron bombardment of lighter elements, and hence the heaviest element that can be prepared in macroscopic quantities. The chemical properties of fermium have been studied solely using tracer amounts and innovative experimental techniques are required. Fermium metal has not been prepared, however measurements have been made on fermium alloys with rare earth metals and a number of predictions have been made. It was deduced that fermium metal prefers a divalent state but with modest compression can form a trivalent state. Other measurements on mixed fermium alloys and compounds include the magnetic moment, inner-shell binding energies, x-ray energies, sublimation enthalpy, etc.

The chemistry of fermium is typical of the late actinides, with a dominance of the +3 oxidation state but also a tendency toward an accessible +2 oxidation state. In the solid state no pure fermium compounds have been prepared, however Fm(III) has been studied by co-crystallization techniques as a trace component in a rare earth matrix with the same charge. Fermium co-precipitates with rare earth fluorides and hydroxides. In aqueous solution, fermium exists in solution as the Fm3+ ion, which has a hydration number of 16.9 and an acid dissociation constant of 1.6 × 10-4 (pKa = 3.8). Fm3+ 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 lighter actinides. It also forms complexes with ligands such as chloride or nitrate and, again, these complexes appear to be more stable than those formed by einsteinium or californium. Bonding in the heavier actinides is mostly ionic in character and the ionic radius of the Fm3+ ion is smaller than the preceding An3+ ions because of the actinide contraction. This is the result of a higher effective nuclear charge of fermium, and thus fermium forms shorter and stronger metal–ligand bonds. In the heavier actinides there is an increasing tendency to form a divalent ion that emerges at einsteinium. Fm3+ can be readily reduced to stable Fm2+ using moderately strong reducing agents such as samarium(II) chloride. In aqueous media, the Fm(III)/Fm(III) redox couple has been investigated via radio-electrochemistry and other techniques. The electrode potentials have been estimated to be similar to that of the ytterbium redox couple. The redox potentials for the various fermium couples have been measured and/or estimated by various workers: Fm3+ → Fm2+ (- 1.15 V); Fm2+ → Fm0 (-2.37 V), all versus the Normal Hydrogen Electrode.

Fermium was discovered by a team of scientists led by Albert Ghiorso in 1952 while studying the radioactive debris produced by the detonation of the first hydrogen bomb. The isotope they discovered, fermium-255, has a half-life of about 20 hours and was produced by combining 17 neutrons with uranium-238, which then underwent eight beta decays. Today, fermium is produced though a lengthy chain of nuclear reactions that involves bombarding each isotope in the chain with neutrons and then allowing the resulting isotope to undergo beta decay. Fermium's most stable isotope, fermium-257, has a half-life of about 100.5 days. It decays into californium-253 through alpha decay or decays through spontaneous fission.

Fermium, element 100, is the eighth transuranium element of the actinide series and is named after the Italian physicist and Nobel Laureate Enrico Fermi. Element 100 was first discovered in 1952 in the fallout from the 10-megaton "Ivy Mike" nuclear test in the south Pacific the first successful test of a hydrogen fusion bomb. Researchers identified a new Pu-244 isotope found on filter papers on drone aircraft flown through the fallout. They determined that it could only have formed by the unexpected absorption of six neutrons by uranium-238 followed by successive beta-decays. At the time, the absorption of neutrons by a heavy nucleus was thought to be a rare process, but the identification of Pu-244 raised the possibility that still more neutrons could have been absorbed by the uranium nuclei leading to additional new elements.

Element 99, einsteinium was discovered almost immediately on other filter papers by Albert Ghiorso and co-workers at the Lawrence Berkeley Laboratory in collaboration with Argonne and Los Alamos National Laboratories, demonstrating that 15 neutrons were captured by U-238! The subsequent discovery of fermium required more material, as the yield of element 100 was expected to be at least an order of magnitude lower than that of einsteinium. So, contaminated coral from ground zero on Eniwetok atoll was shipped to Berkeley for processing and analysis. About two months after the Ivy-Mike test, a new activity was isolated emitting high-energy α-particles (7.1 MeV) with a half-life of about 1 day. It was the β- decay daughter of an isotope of einsteinium, and it had to be an isotope of element 100. : It was identified as 255Fm (half-life 20.07 hours). The discovery of the new elements, and the new data on neutron capture, was kept secret on the orders of the U.S. Military until 1955 due to Cold War tensions. Later the Berkeley team was able to prepare elements 99 and 100 in the lab by neutron bombardment of Pu-239 in a cyclotron. They published this work in 1954, with the disclaimer that these were not the first studies that had been carried out on the element. The 'Ivy Mike' studies were later declassified and published in 1955. Meanwhile, a group at the Nobel Institute for Physics in Stockholm independently claimed discovery of element 100 by producing an isotope with a 30-minute half-life and published their work in May 1954. Nevertheless, the historical precedence of the Berkeley team was generally recognized, and with it the prerogative to name the new element in honor of the recently deceased Enrico Fermi, the developer of the first artificial self-sustained nuclear reactor.