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Berkelium

Atomic Number: 97 Atomic Radius: 244 pm (Van der Waals)
Atomic Symbol: Bk Melting Point: 1050 °C
Atomic Weight: 247 Boiling Point: --
Electron Configuration: [Rn]7s25f9 Oxidation States: 4, 3

History

Berkelium, the eighth member of the actinide transition series, was first produced in 1949 by Thompson, Ghiorso, and Seaborg via accelerator bombardment of 241Am with high energy alpha particles. This generated a new electron-capture activity eluting on a chromatography column just ahead of curium. This activity was assigned to an isotope of element 97 with mass number 243. It was named berkelium after Berkeley, California, the city of its discovery. Initial investigation of its chemical properties were limited to tracer experiments (ion exchange and co-precipitation) but these were sufficient to establish the stability of Bk(III) and the accessibility of Bk(IV) ions in aqueous solution and provide an estimate of the electrochemical potential of the Bk(IV)/Bk(III) couple.

A complete study of an element is not possible by tracer methods alone, so a campaign was initiated in 1952 for long-term irradiation of about 8 grams of 239Pu in a nuclear reactor in Arco, Idaho to provide macro amounts of berkelium. In 1958 about 0.6 micrograms of 249Bk with a half-life of 330 days was recovered, separated, and purified by Cunningham et al. who determined the absorption spectrum in aqueous solution and measured the magnetic susceptibility of Bk(III). The first structural determination of a berkelium compound was in 1962. Four X-ray diffraction lines were obtained from 4 nanograms of berkelium-249 dioxide and indexed as face centered cubic. The first bulk (> 1 microgram) samples of berkelium metal were prepared in 1969 by reduction of BkF3 with lithium metal vapor at 1300 K by Haire and Peterson et al. Bk metal issilvery in appearance, easily soluble in dilute mineral acids, and rapidly oxidized by air or oxygen at elevated temperatures to form the oxide. The metal exhibits two crystal forms: double hexagonal closest packed (dhcp) and face centered cubic (fcc). Numerous alloys and compounds of berkelium have been prepared and studied including hydrides, oxides, halides, chalcogenides, pnictides, oxalates, oxychlorides, organometallic, and coordination compounds to name a few. Berkelium oxidation states Bk(0), Bk(III), and Bk(IV) are known in bulk and some evidence has been offered for the existence of Bk(II) but there is only speculation on the possible existence of Bk(V) ions.

Fourteen isotopes of berkelium are now known and have been synthesized from mass number 238 to 251. As with other actinide elements, berkelium tends to accumulate in the skeletal system. Because of its rarity, berkelium presently has no commercial use, however, with its relatively long half-life and availability in microgram quantities, Bk-249 is used extensively as a target to synthesize heavier elements by charged particle bombardment. Berkelium is the first member of the second half of the actinide series and as such, studies of the physicochemical properties of this element enables more accurate extrapolations to the behavior of the heavier elements for which studies are severely limited by scarcity of material, very short half-lives, and intense radioactivity.

Further reading: D. E. Hobart and J. R. Peterson (2006) "Berkelium," Chapter 10 in The Chemistry of the Actinide and Transactinide Elements, Third Edition, L. R. Morss, J. Fuger, and N. M. Edelstein, Eds, Springer Publishers.

This element reviewed and Updated by David Hobart, Los Alamos National Laboratory 2011

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