Berkelium (Bk)

Berkelium refuses to remain berkelium long enough to be studied properly. The only isotope available in weighable amounts is berkelium-249, whose half-life is 330 days, and its decay product is californium — so a purified sample starts accumulating a different element immediately, at roughly a fifth of a per cent per day. Every berkelium measurement ever published carries a correction for how much of the sample had already stopped being berkelium.

A Swedish village, by way of a Californian city

Element 97 was made in December 1949 by Stanley Thompson, Albert Ghiorso and Glenn Seaborg, who bombarded americium with alpha particles in the Berkeley 60-inch cyclotron and separated a new activity by ion exchange. The hard part had been upstream: accumulating enough americium to shoot at in the first place, which had taken years of reactor irradiation.

The name came from the actinide–lanthanide correspondence. The element directly above 97 in the lanthanides is terbium, whose name comes from Ytterby, the Swedish village whose quarry put its own name on four separate elements. Naming element 97 after Berkeley made the same gesture: an element named for the place that produced it.

The choice drew a mild rebuke from The New Yorker, which suggested the discoverers had wasted an opportunity — they should have called 97 and 98 universitium and ofium, leaving berkelium and californium available for whoever came next. Seaborg's group replied in the same spirit, saying their real worry had been that a group in New York might find the elements first and name them newyorkium and manhattanium.

The +4 state that runs in the family

Berkelium's chemistry is where the terbium parallel stops being a naming convenience and starts being a prediction that came true.

Almost every actinide beyond americium is locked into the +3 oxidation state, for the same reason the late lanthanides are: the f electrons sit too deep to be given up cheaply. Berkelium is the exception. It reaches +4 readily in aqueous solution and holds it, which is what makes berkelium separable from its neighbours at all — an element in a different oxidation state can be pulled out of a mixture where an element in the same one cannot. Terbium does exactly the same thing one row up, for exactly the same reason: removing one more electron leaves a half-filled f shell, and the stability of that half-filled shell pays for the ionisation.

This is a case where the periodic table earned its keep in territory nobody had visited. The oxidation state was expected from the analogy before anyone had a milligram to test it on.

Berkelium's behaviour is not, however, entirely obedient. A 2016 study in Science led by Thomas Albrecht-Schmitt's group characterised berkelium coordination compounds in solution and in the solid state and found the element deviating from the smooth trend the rest of the series follows — evidence that spin–orbit coupling is beginning to reshape actinide chemistry around element 97, rather than waiting until the end of the row.

Two isotopes, and the wrong one is the long-lived one

Twenty isotopes of berkelium are known. The longest-lived is berkelium-247, at 1,380 years, and it is almost entirely unobtainable: it lies on the neutron-poor side of the isotope chart, so no amount of neutron irradiation in a reactor will produce it. Reaching it requires charged-particle bombardment, which produces atoms rather than milligrams.

What is actually available is berkelium-249, which is made the slow way. Plutonium targets sit in Oak Ridge's high-flux reactor for a year or more, capturing neutron after neutron and beta-decaying up the chart, and the berkelium is separated out of a mixture dominated by californium. A campaign of that kind yields tens of milligrams. That is the world supply, and the figures quoted for its value — hundreds of millions of dollars a gram — are notional, because nobody has ever had a gram.

What a 330-day clock actually costs

The half-life dictates the logistics of every experiment involving element 97. Berkelium cannot be stockpiled; it has to be used within months of separation or it is gone. Any programme that needs it has to align a reactor campaign, a chemical separation, a target fabrication and an accelerator run inside a window shorter than a year, with three institutions and often two countries involved.

That constraint is why berkelium's one real application is a single, narrow one: it is the target material for element 117, and putting it under a calcium-48 beam is the only route to that element anyone has demonstrated. Berkelium does not hold that job by rank. Californium sits one square higher, is made in larger amounts, and is the workhorse target everywhere it will serve — the element 118 work ran on it. Berkelium's turn came because tennessine's proton count admits no substitute, which is a much less comfortable position for a supply chain to be in. Every atom of tennessine that has ever existed began as an atom of berkelium.

Beyond that, the element does essentially nothing. There is no berkelium alloy, no berkelium compound in a catalogue, no instrument that uses it. The metal was first prepared in 1969, it is silvery and soft, its crystal packing is the same double-hexagonal arrangement its neighbours adopt, and its existence is justified almost entirely by what it can be turned into.