Fermium

einsteinium ← fermium → mendelevium Er ↑ Fm ↓ –

Atomic properties Atomic number 100 Electron configuration [Rn] 5f12 7s2 Physical properties Melting point 1125 K (850 °C) Chemical properties Electronegativity 1.3 (Pauling) Ionization energy see text Atomic radii Metallic radius 196 pm Ionic radius 92 pm (Fm3+) Thermodynamic properties Enthalpy change of atomization 142(13) kJ mol−1 Entropy change of atomization 98(13) J K−1 mol−1 Miscellaneous CAS number 7440-72-4 Where appropriate, and unless otherwise stated, data are given for 100 kPa (1 bar) and 298.15 K (25 °C).

Fermium (symbol: Fm) is a synthetic chemical element and a member of the actinoid series. It is the heaviest element than can be formed by neutron bombardment of lighter elements, and hence the last element that can be prepared in macroscopic quantities.

It was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Nobel laureate Enrico Fermi, one of the pioneers of nuclear physics. Its chemistry is typical of the late actinoids, with a preponderance of the +3 oxidation state but also an accessible +2 oxidation state.

Isotopes

There are 19 isotopes of fermium listed in NUBASE 2003, with A = 242–260, of which ²⁵⁷Fm is the longest-lived with a half-life of 100.5 days. The neutron-capture product of fermium-257, ²⁵⁸Fm, undergoes spontaneous fission with a half-life of just 370(14) microseconds; ²⁵⁹Fm and ²⁶⁰Fm are also unstable with respect to spontaneous fission (t_½ = 1.5(3) s, 4 ms respectively). This means that neutron capture cannot be used to create nuclides with a mass number greater than 257. As ²⁵⁷Fm is an α-emittor, decaying to 25398Cf, fermium is also the last element that can be prepared by a neutron-capture process.

Discovery

Fermium was first discovered in the fallout from the 'Ivy Mike' nuclear test (1 November 1952), the first successful test of a hydrogen bomb. Initial examination of the debris from the explosion had shown the production of a new isotope of plutonium, 24494Pu: this could only have formed by the absorption of six neutrons by a uranium-238 nucleus followed by two β⁻ decays. At the time, the absorption of neutrons by a heavy nucleus was thought to be a rare process, but the identification of 24494Pu raised the possibility that still more neutrons could have been absorbed by the uranium nuclei, leading to new elements. Element 99 (einsteinium) was quickly discovered on filter papers which had been flown through the cloud from the explosion (the same sampling technique that had been used to discover 24494Pu), specifically the isotope 25499Es resulting from the capture of 16 neutrons by uranium-238 followed by seven β⁻ decays. The discovery of fermium (Z = 100) 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 (7.1 MeV) 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 (t_½ = 20.07(7) hours).

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. Nevertheless, the Berkeley team were 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 (t_½ = 30 minutes) by bombarding a 23892U 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 the recently deceased Enrico Fermi, the developer of the first artificial self-sustained nuclear reactor.

Production

Fermium is produced by the bombardment of lighter actinoids with neutrons in a nuclear reactor. 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. 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 and microgram quantities of fermium can be prepared for specific experiments. The quantities of fermium produced in 20–200 kiloton thermonuclear explosions is believed to be of the order of milligrams, although it is mixed in with a huge quantity of debris; 40 picograms of ²⁵⁷Fm was recovered from 10 kilograms of debris from the 'Hutch' test (16 July 1969).

After production, the fermium must be separated from other actinoids and from lanthanoid 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 (t_½ = 20.07(7) hours) as this isotope can be easily be isolated as required as the decay product of 25599Es (t_½ = 39.8(12) days).

Chemistry

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 × 10⁻⁴ (pK_a = 3.8). Fermium(3+) 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 actinoids. It also forms anionic complexes with ligands such as chloride or nitrate and, again, these complexes appear to be more stable than those formed by einsteinium or californium. It is believed that the bonding in the complexes of the later actinoids is mostly ionic in character: the Fm³⁺ ion is expected to be smaller than the preceeding An³⁺ ions because of the higher effective nuclear charge of fermium, and hence fermium would be expected to form shorter and stronger metal–ligand bonds.

Fermium(III) can be fairly easily reduced to fermium(II), for example with samarium(II) chloride, with which fermium coprecipitates. 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 Fm²⁺/Fm⁰ couple has an electrode potential of −2.37(10) V based on polarographic measurements.

Other properties

The electron configuration of fermium was originally determined by measurement of the magnetic moment of neutral ²⁵⁴Fm atoms: the value of g_j = 1.160 52(14) is only compatible with the ³He6 level of the [Rn] 5f¹² 7s² configuration, and this ground state has since been confirmed by atomic spectroscopy. The atomic spectrum also placed an upper limit on the first ionization energy (which has not been directly determined) of 54 250 cm⁻¹ or 6.73 eV: a theoretical estimate of the first ionization energy is 6.5 eV, while consideration of the measured first ionization energies of berkelium californium and einsteinium suggests a value for fermium of 6.45 eV (622 kJ mol⁻¹).

Although fermium metal has not been prepared as pure samples, several studies on alloys or films suggest that only the two 7s electrons participate in metallic bonding, as is known to be the case for einsteinium: the energy required to promote an electron from the 5f levels to the 6d levels appears to be more than would be gained from the additional bonding. The most direct evidence comes from the measurement of fermium partial pressure above Fm–Sm and (Fm,Es)–Yb alloys for the temperature range 642–905 K (369–632 °C), allowing the calculation of the enthalpy change of sublimation, Δ_subH = 142(13) kJ mol⁻¹. The value of Δ_subH allowed estimates of the metallic radius (198 pm) and the melting point (1125 K, 850 °C) by comparison with the known properties of einsteinium, ytterbium and europium. The estimated metallic radius, which agrees closely with theoretical calculations, implies a solid density of about 9.2 g cm⁻³, higher than that of einsteinium but much lower than those of earlier actinoids.

Notes and references

Further reading

• Hulet, E. K. Chemistry of the Heaviest Actinides: Fermium, Mendelevium, Nobelium, and Lawrencium. In Lanthanide and Actinide Chemistry and Spectroscopy; Edelstein, Norman M., Ed.; American Chemical Society: Washington, D.C., 1980; Chapter 12, pp 239–63. ACS Symposium Series, Vol. 131. ISBN 9780841205680. DOI: 10.1021/bk-1980-0131.ch012.
• Seaborg, Glenn T. The transcalifornium elements. J. Chem. Educ. 1959, 36 (1), 38–44. DOI: 10.1021/ed036p38.

External links

• WebElements