Periodic Table of Elements: Los Alamos National Laboratory

Fermium is named after Enrico Fermi.

Fermium

Atomic Number:	100	Atomic Radius:	--
Atomic Symbol:	Fm	Melting Point:	1527 °C
Atomic Weight:	257	Boiling Point:	--
Electron Configuration:	[Rn] 5f¹² 7s²	Oxidation States:	2, 3

History

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.

Isotopes

A total of 21 known isotopes of fermium exist with atomic weights from 242 to 260, including 2 that are metastable. Fermium-257 is the longest-lived with a half-life of 100.5 days. Other relatively long-lived isotopes include Fm-253 (3 days), Fm-252 (25.4 hours) and Fm-255 (~20 hours). Fm-250, with a half-life of 30 minutes, was shown to be a decay product of nobelium, element 102 and the chemical identification of the isotope ²⁵⁰Fm confirmed the production and discovery of element 102. All the remaining isotopes of fermium have half-lives ranging from 30 minutes to less than a millisecond. The neutron-capture product of fermium-257, ²⁵⁸Fm, undergoes spontaneous fission with a half-life of just 370 microseconds; ²⁵⁹Fm and ²⁶⁰Fm are also unstable with respect to spontaneous fission (t½ = 1.5 s and 4 ms respectively). This means that the neutron capture production chain essentially terminates at mass number 257 because of the very short spontaneous fission half-lives of the heavier isotopes.

Production

Because of the short half-life of all fermium isotopes, all that may have been present on the Earth during its formation has long since decayed away. Einsteinium and fermium did occur in the natural nuclear fission reactor at Oklo, but no longer exist. Fermium is produced as the result of multiple neutron captures in lighter elements, such as uranium and curium, followed by successive beta decays. The probability of such events increases with increased neutron flux, and nuclear explosions are the most powerful neutron sources on Earth. Fermium is also produced by the bombardment of lighter actinides with neutrons in nuclear reactors or accelerators. Fermium-257 is the heaviest isotope that is obtained via neutron capture, and can only be produced in nanogram quantities. The major source is the 85 MW High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory in Tennessee, USA. In a HFIR "campaign", tens of grams of curium are irradiated to produce heavier actinides and picogram quantities of fermium. The quantities of fermium produced in 20–200 kiloton thermonuclear explosions are believed to be of the order of milligrams, although it is mixed in with a huge quantity of debris. Forty picograms of ²⁵⁷Fm were recovered from 10 kilograms of debris from the 'Hutch' nuclear test in 1969. After production, fermium must be separated from debris and a host of other actinides and lanthanide fission products by solvent extraction, ion exchange, etc.). The annual reactor production of fermium-257 is in the picogram range. However, pure ²⁵⁵Fm (half-life 20 hours) can be easily isolated by "milking" the beta-decay daughter of pure ²⁵⁵Es (half-life 39.8 days).

Properties

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.

A fermium-ytterbium alloy was used for measuring the enthalpy of vaporization of fermium metal. The fermium content of this fermium alloy is 4 × 10⁻⁵ weight percent (Courtesy Oak Ridge National Laboratory)

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 (pK_a = 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.

Uses

Owing to the minute amounts of fermium produced and all of its isotopes having relatively short half-lives, there are currently no uses for it outside of basic scientific research that expands knowledge of the rest of the periodic table.