Berkelium

curium ← berkelium → californium Tb ↑ Bk ↓ –

Atomic properties Atomic number 97 Electron configuration [Rn] 5f9 7s2 Physical properties Melting point 1272(22) K (999 °C) Boiling point 2900(50) K (2625 °C) Density 14.78 g cm−3 Chemical properties Electronegativity 1.3 (Pauling) Ionization energy 6.1979(2) eV 598.01(2) kJ mol−1 Atomic radii Metallic radius 170 pm Ionic radius 96 pm (Bk3+, Oh) 83 pm (Bk4+, Oh) Thermodynamic properties 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).

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 (t_½ = 4.5(2) hours) was separated by ion exchange chromatography, where it elutes just ahead of curium, its β⁺-decay product.

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.

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 (t_½ = 330(4) days), is still the only way of producing weighable amounts of the element. 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: In total, just over one gram of ²⁴⁹Bk has been produced at Oak Ridge since 1967.

There is no use for any isotope of berkelium outside of basic scientific research. Berkelium-249 has been used as a target nuclide to prepare still heavier elements, such as lawrencium, rutherfordium and bohrium. 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.

Isotopes

There are 18 isotopes of berkelium listed in NUBASE 2003, with A = 235–254. 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.

Berkelium metal

Berkelium metal was first prepared in 1969 by the reduction of berkelium(III) fluoride with lithium vapour. It can also be prepared by the reduction of berkelium(IV) oxide with thorium or lanthanum. 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. 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. The melting point itself is difficult to determine for the same reasons, but a weighted mean of two consistent reports 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. It dissolves rapidly in aqueous mineral acids, liberating H₂ and forming solutions of the Bk³⁺ ion.

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.

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.. Berkelium(II) oxide, BkO, has been claimed but is not well characterized: it may be a nitride or a mixed oxide-nitride.

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.

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

External links

• WebElements