Einsteinium (Es)

Einsteinium was not made in a laboratory. It was made in a fireball over the Pacific and collected afterwards on filter paper.

On 1 November 1952 the United States detonated Ivy Mike at Enewetak Atoll, the first full-scale thermonuclear test. Aircraft flew through the cloud with filters mounted on their wings, and coral was scraped from the atoll. When Albert Ghiorso's group at Berkeley worked through that debris, together with teams at Argonne and Los Alamos, they found two new elements in it — the heaviest things anyone had ever detected, sitting in ash.

Seventeen neutrons in a microsecond

The mechanism was not the slow neutron capture that reactors do. In a reactor a uranium nucleus picks up one neutron, waits, beta-decays, picks up another. In the microseconds of a thermonuclear explosion, the neutron flux is so intense that a single uranium-238 nucleus can absorb fifteen, sixteen, seventeen neutrons before it has time to decay at all. What emerges is a grotesquely neutron-rich uranium isotope, which then walks up the periodic table by successive beta decays — each one converting a neutron into a proton — until it lands somewhere in the nineties.

That was how einsteinium-253 appeared. It is a route to heavy elements that works only in conditions no laboratory can reproduce, and it produced element 99 and element 100 simultaneously and by accident.

Three years under seal

The result could not be published. The isotopic distribution in the debris was a direct readout of the neutron flux the device had produced, and the neutron flux was among the most closely guarded numbers of the early Cold War. The discovery stayed classified until 1955.

The delay has an awkward consequence for a question that came up decades later. Albert Einstein was alive when element 99 was found, and dead — he died in April 1955 — by the time the name was proposed and the work released. When IUPAC later tried to establish that elements should not be named after living people, einsteinium was repeatedly cited as the precedent that made the rule untenable. Whether it really is one depends on which date you count, and there is no clean answer: the element was discovered in Einstein's lifetime, but the naming was not.

Einstein himself, who had spent his last years arguing against nuclear weapons, was never told that an element had been extracted from the debris of one.

An element that wrecks its own crystals

Einsteinium's most distinctive property is self-destruction, and it comes from a single number. Einsteinium-253 has a half-life of 20.47 days, which is short enough that a gram of it would release energy on the order of a kilowatt — heat, from alpha decay alone, in a sample you could lose on a bench.

Nobody has ever had a gram. But even in microgram amounts the consequences are visible and measurable:

• The material glows, warmed and excited by its own decay.
• Alpha particles knock atoms out of position faster than the lattice can heal, so crystalline einsteinium compounds turn amorphous over weeks. X-ray diffraction patterns fade while the experiment is running.
• Every measured property drifts, because the sample is simultaneously losing einsteinium, gaining its decay products, and being structurally damaged by the process.

There is a related practical problem. Einsteinium-253 decays to berkelium-249, and einsteinium is the heaviest element that has ever been prepared in a quantity large enough to see. About ten micrograms of the metal were obtained in 1961 and weighed on a purpose-built magnetic balance — which remains the record for anything past element 99.

Two hundred nanograms, and a bond that came out short

For more than forty years after the last serious chemical study, essentially nothing new was learned about how einsteinium bonds. The obstacles were the supply — a few tens of micrograms a year from Oak Ridge, and only ever as a by-product of a larger campaign — and the self-damage that ruins a sample mid-measurement.

In 2021 a collaboration led by Rebecca Abergel at Berkeley Lab and Stosh Kozimor at Los Alamos published the first structural and spectroscopic characterisation of an einsteinium coordination complex, in Nature. They worked with less than 200 nanograms of einsteinium-254, bound to an organic chelating ligand, and measured the einsteinium–oxygen bond distance at 2.38 ångströms.

Two results were surprises. The bond was shorter than the smooth actinide trend predicts, and the complex luminesced in the opposite direction to expectation — a blue shift where every model said red. Both point the same way: at the far end of the actinide series, the 5f electrons stop behaving like a tidy continuation of the elements below, and relativistic effects begin to reshape the chemistry. The sample decayed measurably during the experiment, and the isotope had to be apportioned between the structural and spectroscopic work because there was not enough for both to be done comfortably.

Its one real job

Einsteinium's only application is as raw material for the elements above it, and the odd thing about that career is how short it turned out to be. Element 99 supplied the target that yielded element 101 in 1955 — an experiment that could proceed on an invisible smear of einsteinium-253 because it had been designed from the start to work on single atoms, and which is told properly on the mendelevium page. After that the trail goes cold.

The reason is arithmetic about supply rather than any failure of nerve. A target for the calcium-48 campaigns that filled the top of the table is a deposit spread over several square centimetres, and it swallows some hundreds of times the world's entire annual output of einsteinium — a few tens of micrograms of a nuclide that is half gone within a year of leaving the reactor. Californium, one square lower, is the heaviest element that can be supplied on that scale, and it marks where the practical ceiling of target-making actually falls. Einsteinium's industrial record is therefore a single experiment seventy years old: an element with no use except as a step toward the next one, a step it has been used for exactly once.