Berkelium

Berkelium was first synthesised in December 1949 at the University of California, Berkeley, by Stanley Thompson, Albert Ghiorso and Glenn Seaborg, by bombarding americium with alpha particles. It was named after the city of Berkeley, home of the laboratory where so many of the transuranium elements were made. Berkelium is a silvery, radioactive metal, produced only in minute quantities and used chiefly as a target for making still heavier elements. Made in only milligram amounts in the world's most powerful reactors, berkelium has no use outside research, but it is a precious target material: bombarded with calcium ions it yielded tennessine, element 117, one of the last gaps in the periodic table to be filled.

A single gram of berkelium is estimated to cost on the order of hundreds of millions of dollars to produce, since making even the milligram-scale quantities used to synthesise tennessine required Oak Ridge National Laboratory's High Flux Isotope Reactor to irradiate target material continuously for many months, followed by a lengthy chemical purification process, making the 2009 batch used for that experiment one of the most expensive small quantities of matter ever deliberately manufactured.

Berkelium's name honours the city and university of Berkeley, California, the great workshop of element-making where Seaborg's group created one new element after another. The choice was made by analogy with the element directly above it in the periodic table, terbium, which is named for the Swedish village of Ytterby; just as terbium honoured the place where the lanthanides were unravelled, berkelium honoured the place where the actinides were being built. The parallel is a small joke of the discoverers, matching a place-named lanthanide with a place-named actinide beneath it.

Berkelium has no ancient, mythological or alchemical tradition, being a synthetic element of the mid-twentieth century; its significance is as a stepping-stone, a rare and precious target material from which the heaviest elements of all, including tennessine, have since been forged. Chemists studying berkelium have found it can, unusually among the later actinides, be stabilised in a plus-four oxidation state much like its lanthanide analogue cerium, a chemical echo across the two f-block series that helps researchers test theoretical models of how electrons behave in the heaviest, least-studied corners of the periodic table.