Berkelium (97)

Berkelium is a radioactive rare earth metal, named after the University of California at Berkeley (USA). Apparently, berkelium tends to accumulate in the skeletal system. It is of no commercial importance and only a few of its compounds are known.

Name: Berkelium Symbol: Bk Atomic number: 97 Atomic weight: [ 247 ] Standard state: solid at 298 K CAS Registry ID: 7440-40-6

Group name: Actinoid Period in periodic table: 7 (actinoid) Block in periodic table: f-block Color: unknown, but probably metallic and silvery white or grey in appearance Classification: Metallic

Historical information

Although very small amounts of berkelium were possibly produced in previous nuclear experiments, it was first intentionally synthesized, isolated and identified in December 1949 by Glenn T. Seaborg, Albert Ghiorso and Stanley G. Thompson. They used the 60-inch cyclotron at the University of California, Berkeley. Similar to the nearly simultaneous discovery of americium (element 95) and curium (element 96) in 1944, the new elements berkelium and californium (element 98) were both produced in 1949–1950.

The name choice for element 97 followed the previous tradition of the Californian group to draw an analogy between the newly discovered actinide and the lanthanide element positioned above it in the periodic table. Previously, americium was named after a continent as its analogue europium, and curium honored scientists Marie and Pierre Curie as the lanthanide above it, gadolinium, was named after the explorer of the rare earth elements Johan Gadolin. Thus the discovery report by the Berkeley group reads: "It is suggested that element 97 be given the name berkelium (symbol Bk) after the city of Berkeley in a manner similar to that used in naming its chemical homologue terbium (atomic number 65) whose name was derived from the town of Ytterby, Sweden, where the rare earth minerals were first found." This tradition ended on berkelium, though, as the naming of the next discovered actinide, californium, was not related to its lanthanide analogue dysprosium, but after the discovery place.

The most difficult steps in the synthesis of berkelium were its separation from the final products and the production of sufficient quantities of americium for the target material. First, americium (²⁴¹Am) nitrate solution was coated on a platinum foil, the solution was evaporated and the residue converted by annealing to americium dioxide (AmO₂). This target was irradiated with 35 MeV alpha particles for 6 hours in the 60-inch cyclotron at the Lawrence Radiation Laboratory, University of California, Berkeley. The (α,2n) reaction induced by the irradiation yielded the ²⁴³Bk isotope and two free neutrons

The first visible amounts of a pure berkelium compound, berkelium chloride, were produced in 1962, and weighed just 3 billionth of a gram (3.0 x 10^-10 g or 0.0000000003 g). Very, very tiny!

Physical properties

• Melting point: 1259 [or 986 °C (1807 °F)] K
• Boiling point: no data K
• Density of solid: 14780 kg m^-3

Orbital properties

• Ground state electron configuration:  [Rn].5f⁹.7s²
• Shell structure:  2.8.18.32.27.8.2
• Term symbol:   ⁶H_15/2
• Pauling electronegativity: 1.3 (Pauling units)
•  First ionization energy: 601 kJ mol^-1
•  Second ionization energy: no data kJ mol^-1

Occurrence

All berkelium isotopes have a half-life far too short to be primordial (existing in original form since before the formation of the Earth). Thusly, all primordial berkelium has decayed by now and any present on the planet has been created in laboratory.
On Earth, berkelium is mostly concentrated in certain areas, which were used for the atmospheric nuclear weapons tests between 1945 and 1980, as well as at the sites of nuclear incidents, such as the Chernobyl disaster, Three Mile Island accident and 1968 Thule Air Base B-52 crash. The first US hydrogen bomb, a 62 ton fusion bomb (code name Ivy Mike), was tested at the Enewetak Atoll on 1 November, 1952. Analysis of the testing site debris revealed high concentrations of various actinides, including berkelium. For reasons of military secrecy, this result was published only in 1956.
Nuclear reactors produce mostly, among the berkelium isotopes, berkelium-249. During the storage and before the fuel disposal, most of it beta decays to californium-249. The latter has a half-life of 351 years, which is relatively long when compared to the other isotopes produced in the reactor, and is therefore undesirable in the disposal products.
A few atoms of berkelium can be produced by neutron capture reactions and beta decay in very highly concentrated uranium-bearing deposits, thus making it the rarest naturally occurring element.

Health Issues

Little is known about the effects of berkelium on human body, and analogies with other elements may not be drawn because of different radiation products (electrons for berkelium and alpha particles, neutrons, or both for most other actinides). The low energy of electrons emitted from berkelium-249 is less than 126 keV, (kiloelectonVolt- a unit of energy equal to approximately 1.6 × 10 −19 J) hinders its detection due to signal interference with other decay processes, but also makes this isotope relatively harmless to humans as compared to other actinides. However, berkelium-249 transforms with a half-life of only 330 days to the strong alpha-emitter californium-249, which is rather dangerous and has to be handled in a glove box in a dedicated laboratory.
Most available berkelium toxicity data originate from research on animals. Upon ingestion by rats, only about 0.01% berkelium ends in the blood stream. From there, about 65% goes to the bones, where it remains for about 50 years, 25% to the lungs (biological half-life about 20 years), 0.035% to the testicles or 0.01% to the ovaries where berkelium stays indefinitely. The balance of about 10% is excreted. In all these organs berkelium might promote cancer, and in the skeletal system its radiation can damage red blood cells. The maximum permissible amount of berkelium-249 in the human skeleton is 0.4 nanograms.