Berkelium

As was the case for the previously discovered transuranium elements, element 97 was first produced via a nuclear bombardment reaction. In December 1949 ion-exchange separation of the products formed by the bombardment of ²⁴¹Am with accelerated alpha particles provided a new electron-capture activity eluting just ahead of curium (Thompson et al., 1950a,b). This activity was assigned to an isotope (mass number 243) of element 97. The new element was named berkelium after Berkeley, California, USA, the city of its discovery, in a manner parallel to the naming of its lanthanide analog, terbium, after Ytterby, Sweden. The initial investigations of the chemical properties of berkelium were limited to tracer experiments (ion exchange and coprecipitation), and these were sufficient to establish the stability of Bk(III) and the accessibility of Bk(IV) in aqueous solution and to estimate the electrochemical potential of the Bk(IV)/Bk(III) couple (Thompson et al., 1950b,c). Because a complete study of the chemistry of an element is not possible by tracer methods alone, a program for long-term neutron irradiation of about 8 g of ²³⁹Pu was initiated in 1952 in the Materials Testing Reactor (Arco, Idaho, USA) to provide macroquantities of berkelium (Cunningham, 1959). In 1958 about 0.6 mg of ²⁴⁹Bk was separated, purified, and used in experiments to determine the absorption spectrum of Bk(III) in aqueous solution and to measure the magnetic susceptibility of Bk(III) (Cunningham, 1959). No Bk(III) absorption was observed over the wavelength range 450–750 nm, but an upper limit of about 20 was set for the molar absorptivity of any Bk(III) absorption in this wavelength region. The magnetic susceptibility, measured from 77 to 298 K with the Bk(III) ions sorbed in a single bead of cation-exchange resin, was found to conform to the Curie-Weiss law with an effective moment of 8.7 μ_B, suggesting a 5f⁸ electronic configuration for the Bk(III) ion. The first structure determination of a compound of berkelium, the dioxide, was carried out in 1962 (Cunningham, 1963). Four X-ray diffraction lines were obtained from 4 ng of BkO₂ and indexed on the basis of a facecentered cubic (fcc) structure with a⁰ = (0.533 ± 0.001) nm. In the intervening years since this initial work to characterize element 97, considerable information about the physicochemical properties of berkelium has been obtained in spite of the rather limited availability and the short half-life (330 days) of ²⁴⁹Bk, the only isotope available in bulk quantities.

The authors have focused this review of the chemistry of berkelium on open literature references in English or English translation, except where it was deemed necessary to cite a research institution report or technical memorandum or personal communication. References to theses, dissertations, and patents are minimal. The biologic and metabolic effects of exposure to and/or ingestion of berkelium on humans and animals have not been reviewed here (see Chapter 31). Also excluded are references dealing with the determination and/or use of the nuclear properties of the various isotopes of berkelium, with the notable exception of a few modern references dealing with the use of 249Bk as a target material for the production of transactinide elements. The references cited herein are not necessarily inclusive or always the original ones, yet they should be adequate to permit the interested reader to access easily the broader literature beyond.