Berkelium

Berkelium was synthesized in December 1949 by Stanley Thompson, Albert Ghiorso, and Glenn Seaborg at the University of California, Berkeley, making it the fifth transuranium element produced in the laboratory. The team bombarded americium-241 with alpha particles in the 60-inch cyclotron, creating a few thousand atoms of berkelium-243. The element is a silvery metal, highly radioactive, and so difficult to produce that it took years before even microgram quantities could be accumulated for study.

Berkelium's primary scientific value lies in its role as a target material for synthesizing heavier elements. In 2009 and 2010, a 22-milligram sample of berkelium-249 — painstakingly produced at Oak Ridge National Laboratory over 250 days — was shipped to a Russian facility and bombarded with calcium-48 ions to create tennessine (element 117). Apart from this role as a stepping stone to undiscovered elements, berkelium serves as a subject of fundamental actinide chemistry research.

Thompson, Ghiorso, and Seaborg first produced berkelium-243 in December 1949 at UC Berkeley's Radiation Laboratory. The element was named after Berkeley, California, the city where it was created — following the precedent set by californium, which came just a year later. The name was also chosen to honor the University of California, Berkeley, where so many transuranium elements had been discovered. Isolation of a weighable amount of berkelium did not occur until 1958, nearly a decade after its discovery, reflecting the immense difficulty of producing the element in quantity.

Berkelium does not occur naturally on Earth. It is produced exclusively in nuclear reactors by bombarding curium or other heavy actinides with neutrons, followed by particle accelerator reactions. Oak Ridge National Laboratory in Tennessee is the primary facility worldwide capable of producing milligram-scale quantities, a process that requires months of continuous irradiation.

Berkelium chemistry is limited by the extreme scarcity of the element, but experiments have characterized its primary +3 oxidation state alongside a relatively stable +4 state — making it unusual among the heavier actinides. Known compounds include berkelium(III) and berkelium(IV) oxides and halides. The accessibility of the +4 state distinguishes berkelium from neighboring curium and californium. Most chemical studies have been performed on microscopic samples using highly sensitive analytical techniques.

• The 22-milligram berkelium-249 sample used to create tennessine in 2010 represented one of the largest quantities of the element ever assembled in one place.
• Berkelium was named after Berkeley, California, making it one of the few elements named after a city rather than a country, scientist, or mythological figure.
• Producing enough berkelium for a single experiment at Oak Ridge requires continuous neutron bombardment in a high-flux reactor for up to 18 months.
• Berkelium-249 has a half-life of only 330 days, so scientists must work quickly after it is produced — the clock starts ticking from the moment irradiation ends.
• Despite being discovered in 1949, berkelium's crystal structure was not determined until 1970 because so little of the element had ever been produced.