Fermium

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einsteinium ← fermium → mendelevium
Er
↑
Fm
↓
–
Atomic properties
Atomic number 100
Electron configuration [Rn] 5f12 6s2
Physical properties[1][Note 1]
Melting point 1125 K (850 °C)
Chemical properties[Note 1]
Electronegativity 1.3 (Pauling)
Ionization energy
6.50 eV
627 kJ mol−1
Atomic radii[1][2][3][Note 1]
Metallic radius 196 pm
Ionic radius 92 pm (Fm3+)
Thermodynamic properties[1]
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).
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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

Main article: Isotopes of fermium

There are 19 isotopes of fermium listed in NUBASE 2003,[4] with A = 242–260,[Note 2] 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 (t½ = 1.5(3) s, 4 ms respectively).[4][Note 2] 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.[5][6]

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.[7] 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.[7] 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 (t½ = 20.07(7) hours).[7]

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.[7] 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.[8][9] The 'Ivy Mike' studies were declassified and published in 1955.[10]

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,[7] 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 (t½ = 30 minutes) by bombarding a 23892U target with oxygen-16 ions, and published their work in May 1954.[11] 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 at the Oak Ridge National Laboratory in Tennessee, USA, which is dedicated to the production of transcurium (Z > 96) elements.[12] 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.[13] However nanogram[14] and microgram[5] 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).[15]

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.[6][16] Smaller cations form more stable complexes with the α-hydroxyisobutyrate anion, and so are preferentially eluted from the column.[6] A rapid fractional crystallization method has also been described.[6][17]

Although the most stable isotope of fermium is 257Fm, with a half-life of 100.5 days, most studies are conducted on 255Fm (t½ = 20.07(7) hours) as this isotope can be easily be isolated as required as the decay product of 25599Es (t½ = 39.8(12) days).[6]

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 (pKa = 3.8).[18][19] 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.[6] 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.[20] It is believed that the bonding in the complexes of the later actionoids 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.[6]

Fermium(III) can be fairly easily reduced to fermium(II).

Notes and references

Notes

  1. ↑ 1.0 1.1 1.2 Many of the properties of fermium are only known through estimation and/or extrapolation. The melting point and metallic radius were estimated on the basis of the enthalpy change of atomization and comparison with divalent lanthanoids; the electronegativity was estimated on the basis of periodic trends; the ionic radius was estimated from the behaviour of Fm3+ α-hydroxyisobutyrate (α-HIB) complexes on ion exchange columns, and agrees well with theoretical calculations. The electron configuration and thermodynamic properties were directly determined.
  2. ↑ 2.0 2.1 The discovery of 260Fm is considered "unproven" in NUBASE 2003.[4]

References

  1. ↑ 1.0 1.1 1.2 Haire, Richard G.; Gibson, John K. The enthalpy of sublimation and thermodynamic functions of fermium. J. Chem. Phys. 1989, 91 (11), 7085–96. DOI: 10.1063/1.457326.
  2. ↑ David, F.; Samhoun, K.; Guillaumont, R.; Edelstein, N. Thermodynamic properties of 5f elements. J. Inorg. Nucl. Chem. 1978, 40 (1), 69–74. DOI: 10.1016/0022-1902(78)80309-1.
  3. ↑ Brüchle, W.; Schädel, M.; Scherer, U. W.; Kratz, J. V.; Gregorich, K. E.; Lee, D.; Nurmia, M.; Chasteler, R. M., et al. The hydration enthalpies of Md3+ and Lr3+. Inorg. Chim. Acta 1988, 146 (2), 267–76. DOI: 10.1016/S0020-1693(00)80619-2.
  4. ↑ 4.0 4.1 4.2 Audi, G.; Bersillon, O.; Blachot, J.; Wapstra, A. H. The NUBASE evaluation of nuclear and decay properties. Nucl. Phys. A 2003, 729, 3–128. doi:10.1016/j.nuclphysa.2003.11.001, <http://amdc.in2p3.fr/nubase/Nubase2003.pdf>.
  5. ↑ 5.0 5.1 Greenwood, Norman N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; p 1463. ISBN 0-08-022057-6.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 Silva, Robert J. Fermium, Mendelevium, Nobelium, and Lawrencium. In The Chemistry of the Actinide and Transactinide Elements, 3rd ed.; Morss, Lester R.; Edelstein, Norman M.; Fuger, Jean, Eds.; Springer: Dordrecht, 2006; Vol. 3, Chapter 13, pp 1621–51. doi:10.1007/1-4020-3598-5_13, <http://radchem.nevada.edu/classes/rdch710/files/Fm%20to%20Lr.pdf>.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 Ghiorso, Albert Einsteinium and Fermium. Chem. Eng. News 2003, 81 (36), <http://pubs.acs.org/cen/80th/einsteiniumfermium.html>.
  8. ↑ Thompson, S. G.; Ghiorso, A.; Harvey, B. G.; Choppin, G. R. Transcurium Isotopes Produced in the Neutron Irradiation of Plutonium. Phys. Rev. 1954, 93 (4), 908. DOI: 10.1103/PhysRev.93.908.
  9. ↑ Choppin, G. R.; Thompson, S. G.; Ghiorso, A.; Harvey, B. G. Nuclear Properties of Some Isotopes of Californium, Elements 99 and 100. Phys. Rev. 1954, 94 (4), 1080–81. DOI: 10.1103/PhysRev.94.1080.
  10. ↑ Ghiorso, A.; Thompson, S. G.; Higgins, G. H.; Seaborg, G. T.; Studier, M. H.; Fields, P. R.; Fried, S. M.; Diamond, H., et al. New Elements Einsteinium and Fermium, Atomic Numbers 99 and 100. Phys. Rev. 1955, 99 (3), 1048–49. DOI: 10.1103/PhysRev.99.1048.
  11. ↑ Atterling, Hugo; Forsling, Wilhelm; Holm, Lennart W.; Melander, Lars; Åström, Björn Element 100 Produced by Means of Cyclotron-Accelerated Oxygen Ions. Phys. Rev. 1954, 95 (2), 585–86. DOI: 10.1103/PhysRev.95.585.2.
  12. ↑ High Flux Isotope Reactor; Oak Ridge National Laboratory, <http://neutrons.ornl.gov/facilities/HFIR/>. (accessed 23 September 2010).
  13. ↑ Porter, C. E.; Riley, F. D., Jr.; Vandergrift, R. D.; Felker, L. K. Fermium Purification Using Teva™ Resin Extraction Chromatography. Sep. Sci. Technol. 1997, 32 (1–4), 83–92. DOI: 10.1080/01496399708003188.
  14. ↑ Sewtz, M.; Backe, H.; Dretzke, A.; Kube, G.; Lauth, W.; Schwamb, P.; Eberhardt, K.; Grüning, C., et al. First Observation of Atomic Levels for the Element Fermium (Z = 100). Phys. Rev. Lett. 2003, 90 (16), 163002. DOI: 10.1103/PhysRevLett.90.163002.
  15. ↑ Hoff, R. W.; Hulet, E. K. Engineering with Nuclear Explosives, 1970; Vol. 2, pp 1283–94.
  16. ↑ Choppin, G. R.; Harvey, B. G.; Thompson, S. G. A new eluant for the separation of the actinide elements. J. Inorg. Nucl. Chem. 1956, 2 (1), 66–68. DOI: 10.1016/0022-1902(56)80105-X.
  17. ↑ Mikheev, N. B.; Kamenskaya, A. N.; Konovalova, N. A.; Rumer, I. A.; Kulyukhin, S. A. Radiokhimiya 1983, 25 (2), 158–61.
  18. ↑ Lundqvist, Robert; Hulet, E. K.; Baisden, T. A. Electromigration Method in Tracer Studies of Complex Chemistry. II. Hydrated Radii and Hydration Numbers of Trivalent Actinides. Acta Chem. Scand., Ser. A 1981, 35, 653–61. DOI: 10.3891/acta.chem.scand.35a-0653.
  19. ↑ Hussonnois, H.; Hubert, S.; Aubin, L.; Guillaumont, R.; Boussieres, G. Radiochem. Radioanal. Lett. 1972, 10, 231–38.
  20. ↑ Thompson, S. G.; Harvey, B. G.; Choppin, G. R.; Seaborg, G. T. Chemical Properties of Elements 99 and 100. J. Am. Chem. Soc. 1954, 76 (24), 6229–36. DOI: 10.1021/ja01653a004.

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