Berkelium

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curium ← berkelium → californium
Tb
↑
Bk
↓
–
Atomic properties
Atomic number 97
Electron configuration [Rn] 5f9 7s2
Physical properties[1][2][3]
Melting point 1272(22) K (999 °C)[Note 1]
Boiling point 2900(50) K (2625 °C)
Density 14.78 g cm−3
Chemical properties[4]
Electronegativity 1.3 (Pauling)[Note 2]
Ionization energy[5][6]
6.1979(2) eV
598.01(2) kJ mol−1
Atomic radii[3][7][8]
Metallic radius 170 pm
Ionic radius 96 pm (Bk3+, Oh)[Note 3]
83 pm (Bk4+, Oh)
Thermodynamic properties[2][9]
Standard entropy 76.2(13) J K−1 mol−1
Enthalpy change of atomization 310(6) kJ mol−1
Enthalpy change of fusion 7.92 kJ mol−1
Miscellaneous
CAS number 7440-40-6
Where appropriate, and unless otherwise stated, data are given for 100 kPa (1 bar) and 298.15 K (25 °C).
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Berkelium (symbol: Bk) is a synthetic chemical element and a member of the actinoid series. It is named after the city of Berkeley, California, the location of the University of California Radiation Laboratory where it was discovered in 1949.

Discovery

Berkelium was first produced in 1949 by the bombardment of an americium-241 target with α-particles: the nuclear reaction is 24195Am(α,2n)24397Bk. The product berkelium-243 (t½ = 4.5(2) hours) was separated by ion exchange chromatography, where it elutes just ahead of curium, its β+-decay product.[10][11][Note 4]

The new element was named after the city of Berkeley, California, by analogy with its lanthanoid homologue terbium, named after the village of Ytterby in Sweden.[11]

Production and use

The first macroscopic quantities (0.8 µg) of berkelium were isolated in 1958 after a six-year irradiation of plutonium-241 with neutrons: this method, which produces the isotope 249Bk (t½ = 330(4) days), is still the only way of producing weighable amounts of the element.[12] 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.[13][Note 5] 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:[16] In total, just over one gram of 249Bk has been produced at Oak Ridge since 1967.[12]

There is no use for any isotope of berkelium outside of basic scientific research.[12] Berkelium-249 has been used as a target nuclide to prepare still heavier elements, such as lawrencium, rutherfordium and bohrium.[12] It is also useful as a source of the isotope 24998Cf, which is used for studies on the chemistry of californium in preference to the more radioactive 25298Cf that is produced in neutron bombardment facilities such as the HFIR.[12][17]

Isotopes

Main article: Isotopes of berkelium

There are 18 isotopes[Note 6] of berkelium listed in NUBASE 2003, with A = 235–254.[18] The only isotope of significant practical importance is berkelium-249, which shows almost pure β− decay with a half-life of 330(4) days. The most stable isotope is berkelium-247, with a half-life (α decay) of 1.38(25) × 103 years.[18]

Berkelium metal

Berkelium metal was first prepared in 1969 by the reduction of berkelium(III) fluoride with lithium vapour.[3] It can also be prepared by the reduction of berkelium(IV) oxide with thorium or lanthanum.[19] It exists as two phases in the solid state: the α-phase, which is the more stable at room temperature, has the dihexagonal close packed structure (ABAC stacking) with a = 341.6(3) pm, c = 1106.9(7) pm; the high-temperature β-phase, which is metastable at room temperature, has the face-centered cubic structure (ABC stacking) with a = 499.7(4) pm.[3] The temperature of the phase transition is very difficult to measure because of the self-heating of samples caused by the intense α-radiation, but is believed to be quite close to the melting point.[12] The melting point itself is difficult to determine for the same reasons, but a weighted mean of two consistent reports[1][2] gives the value of 1272(22) K (999 °C).

Berkelium does not react rapidly with oxygen at room temperature, possibly due to the formation of a protective surface layer of oxide. However, it will react with hydrogen, chalcogens and pnictogens to form binary compounds.[12] It dissolves rapidly in aqueous mineral acids, liberating H2 and forming solutions of the Bk3+ ion.[12]

Compounds

The most common oxidation state for berkelium is +3, although the +4 state is also known both in the solid state and in solution: in this respect, the chemistry of berkelium is more similar to that of cerium than to its direct lanthanoid homologue terbium. The Bk3+ ion is green in aqueous solution (in the absence of coordinating anions), while Bk4+ is yellow in hydrochloric acid solution and orange-yellow in sulfuric acid solution.[12]

Berkelium forms two oxides, BkO2 and Bk2O3, with substantial non-stoichiometry between the two idealized compositions. There is no evidence for an intermediate phase such as the rhombohedral Bk7O12 expected from comparison with lanthanoid chemistry.[20]. Berkelium(II) oxide, BkO, has been claimed[1] but is not well characterized: it may be a nitride or a mixed oxide-nitride.[12]

All four trihalides can be formed by reaction of the appropriate hydrohalous acid with a berkelium oxide. Berkelium(IV) fluoride, BkF4, is the only known binary halide in the +4 oxidation state, although salts of the [BkCl6]2− ion have been prepared.[12]

Two cyclopentadienides of berkelium(III) are known, BkCp3 and [BkCp2Cl]2: they are prepared by the reaction of BkCl3 with molten beryllium cyclopentadienide.[21][22]

Notes and references

Notes

  1. ↑ The melting point quoted here is the weighted mean of the values found by Fahey et al. (1972)[1] and Ward et al. (1982).[2]
  2. ↑ The Pauling electronegativity was estimated from periodic trends rather than being calculated from bond energy data.
  3. ↑ The quoted atomic radii are based on the usual convention that r(O2−, Oh) = 140 pm; on the alternative convention of r(F−, Oh) = 119 pm, the value would be 110 pm for octahedral Bk3+.
  4. ↑ The decay of 243Bk was initially thought to be by electron capture: the product nuclide is the same in both cases, 24396Cm (t½ = 29.1(1) years).
  5. ↑ The Research Institute of Atomic Reactors (NIIAR) in Dimitrovgrad, Russia, is also a producer of transcurium elements.[14] The SM-2 loop reactor at NIIAR has similar power and flux levels to the High Flux Isotope Reactor at Oak Ridge, and so production capacities for transcurium elements are expected to be similar at the two facilities,[15] although the quantities produced at NIIAR are not published.
  6. ↑ This figure does not include nuclear isomers (excited states of the nucleus), many of which are very poorly characterized for the transuranium elements.

References

  1. ↑ 1.0 1.1 1.2 1.3 Fahey, J. A.; Peterson, J. R.; Baybarz, R. D. Some properties of berkelium metal and the apparent trend toward divalent character in the transcurium actinide metals. Inorg. Nucl. Chem. Lett. 1972, 8 (1), 101–7. DOI: 10.1016/0020-1650(72)80092-8.
  2. ↑ 2.0 2.1 2.2 2.3 Ward, John W.; Kleinschmidt, Phillip D.; Haire, Richard G. Vapor pressure and thermodynamics of Bk‐249 metal. J. Chem. Phys. 1982, 77 (3), 1464–68. DOI: 10.1063/1.443975.
  3. ↑ 3.0 3.1 3.2 3.3 Peterson, J. R.; Fahey, J. A.; Baybarz, R. D. The crystal structures and lattice parameters of berkelium metal. J. Inorg. Nucl. Chem. 1971, 33 (10), 3345–51. DOI: 10.1016/0022-1902(71)80656-5.
  4. ↑ Pauling, Linus The Nature of the Chemical Bond, 3rd ed.; Ithaca, NY, 1960. ISBN 0-8014-0333-2.
  5. ↑ Köhler, S.; Deißenberger, R.; Eberhardt, K.; Erdmann, N.; Herrmann, G.; Huber, G.; Kratz, J. V.; Nunnemann, M., et al. Determination of the first ionization potential of actinide elements by resonance ionization mass spectroscopy. Spectrochim. Acta, Part B 1997, 52 (6), 717–26. DOI: 10.1016/S0584-8547(96)01670-9.
  6. ↑ Erdmann, N.; Nunnemann, M.; Eberhardt, K.; Herrmann, G.; Huber, G.; Köhler, S.; Kratz, J. V.; Passler, G., et al. Determination of the first ionization potential of nine actinide elements by resonance ionization mass spectroscopy (RIMS). J. Alloys Compd. 1998, 271–273, 837–40. DOI: 10.1016/S0925-8388(98)00229-1.
  7. ↑ Shannon, R. D.; Prewitt, C. T. Effective ionic radii in oxides and fluorides. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1969, 25, 925–46. DOI: 10.1107/S0567740869003220.
  8. ↑ Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halids and chalcogenides. Acta Crystallogr. A 1976, 32 (5), 751–67. DOI: 10.1107/S0567739476001551.
  9. ↑ Ward, John W.; Hill, H. Hunter An Entropy Correlation for the 4f and 5f Metals: Relation of Electronic Properties to Metallic Radii, Magnetic Transformations and Thermodynamics of Vaporization. In Heavy Element Properties; Müller, Werner; Blank, Hubert, Eds.; Proc. Joint Session, 4th Int. Transplutonium Element Symp. & 5th Int. Conf. on Plutonium and Other Actinides, Baden-Baden, September 1975; North-Holland: Amsterdam, 1976; pp 65–79. ISBN 0720404029.
  10. ↑ Thompson, S. G.; Ghiorso, A.; Seaborg, G. T. Element 97. Phys. Rev. 1950, 77 (6), 838–39. DOI: 10.1103/PhysRev.77.838.2.
  11. ↑ 11.0 11.1 Thompson, S. G.; Ghiorso, A.; Seaborg, G. T. The New Element Berkelium (Atomic Number 97). Phys. Rev. 5, 80, 781–89. DOI: 10.1103/PhysRev.80.781.
  12. ↑ 12.00 12.01 12.02 12.03 12.04 12.05 12.06 12.07 12.08 12.09 12.10 Hobart, David E.; Peterson, Joseph R. Berkelium. In The Chemistry of the Actinide and Transactinide Elements, 3rd ed.; Morss, Lester R.; Edelstein, Norman M.; Fuger, Jean, Eds.; Springer: Dordrecht, the Netherlands, 2006; Vol. 3, Chapter 10, pp 1444–98. doi:10.1007/1-4020-3598-5_10, <http://radchem.nevada.edu/classes/rdch710/files/berkelium.pdf>.
  13. ↑ High Flux Isotope Reactor; Oak Ridge National Laboratory, <http://neutrons.ornl.gov/facilities/HFIR/>. (accessed 23 September 2010).
  14. ↑ Радионуклидные источники и препараты; Research Institute of Atomic Reactors, <http://www.niiar.ru/?q=radioisotope_application>. (accessed 26 September 2010).
  15. ↑ Haire, Richard G. Einsteinium. In The Chemistry of the Actinide and Transactinide Elements, 3rd ed.; Morss, Lester R.; Edelstein, Norman M.; Fuger, Jean, Eds.; Springer: Dordrecht, the Netherlands, 2006; Vol. 3, Chapter 12, pp 1577–1620. doi:10.1007/1-4020-3598-5_12, <http://radchem.nevada.edu/classes/rdch710/files/einsteinium.pdf>.
  16. ↑ 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.
  17. ↑ Haire, Richard G. Californium. In The Chemistry of the Actinide and Transactinide Elements, 3rd ed.; Morss, Lester R.; Edelstein, Norman M.; Fuger, Jean, Eds.; Springer: Dordrecht, the Netherlands, 2006; Vol. 3, Chapter 11, pp 1499–1576. doi:10.1007/1-4020-3598-5_11, <http://radchem.nevada.edu/classes/rdch710/files/californium.pdf>.
  18. ↑ 18.0 18.1 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>.
  19. ↑ Spirlet, J. C.; Peterson, J. R.; Asprey, L. B. Preparation and Purification of Actinide Metals. Adv. Inorg. Chem. 1987, 31, 1–41. DOI: 10.1016/S0898-8838(08)60220-2.
  20. ↑ Turcotte, R. P.; Chikalla, T. D.; Haire, R. G.; Fahey, J. A. Phase behaviour in the berkelium-oxygen system. J. Inorg. Nucl. Chem. 1980, 42 (12), 1729–33. DOI: 10.1016/0022-1902(80)80149-7.
  21. ↑ Laubereau, Peter G.; Burns, John H. Microchemical preparation of tricyclopentadienyl compounds of berkelium, californium, and some lanthanide elements. Inorg. Chem. 1970, 9 (5), 1091–95. DOI: 10.1021/ic50087a018.
  22. ↑ Laubereau, P. G. The formation of dicyclopentadienylberkeliumchloride. Inorg. Nucl. Chem. Lett. 1970, 6 (7), 611–16. DOI: 10.1016/0020-1650(70)80057-5.

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