Flerovium (Fl)

In 1942 a young Soviet physicist serving with the air force spent time in a university library and noticed an absence. Papers on nuclear fission, which had been appearing at a rapid clip in American, British and German journals since 1939, had stopped. Not slowed — stopped, and the names of the authors had stopped appearing on anything else either.

Georgy Flyorov drew the only sensible conclusion: the subject had been classified because somebody was building something. He wrote to Stalin saying so. The letter is generally credited with helping to prompt the Soviet atomic project, and Flyorov went on to found the laboratory at Dubna that made more elements than any other institution on Earth. Element 114 is named for that laboratory, which is itself named for him.

Calcium-48, and why it changed everything

The route that produced elements 114 to 118 was not the lead- and bismuth-target approach that had carried the table from bohrium up to nihonium. It was Flyorov's successor Yuri Oganessian's insight about a particular isotope of calcium.

Calcium-48 has twenty-eight neutrons to its twenty protons. It is stable, it makes up under 0.2% of natural calcium, and it is startlingly neutron-rich for such a light nucleus — both 20 and 28 are magic numbers, and the combination is tightly bound. Fire calcium-48 at a heavy actinide target and two things follow. The fused nucleus starts with far more neutrons than a lighter projectile would supply, which pushes the product toward the region where superheavy nuclei survive longest. And because calcium-48 is doubly magic, the fusion is comparatively gentle.

The catch is supply. Enriching calcium-48 out of natural calcium is enormously expensive, and the world's usable stock is small enough that individual experiments are planned around how many milligrams can be spared. Dubna spent that stock on the last five squares of the table, and element 114 was where it started.

The first attempt came in December 1998: a plutonium-244 target, a joint Dubna and Livermore team, and one decay chain that nothing but element 114 could have started. Element 113 is the one square in this corner that does not belong to the calcium-48 story: Dubna saw it too, as a decay product, but the recognised discovery went to a bismuth target at RIKEN.

The island, and how close this actually is

Flerovium has been entangled with the idea of an "island of stability" for fifty years, and the relationship deserves stating carefully.

Nuclear models have long predicted a region of relatively long-lived superheavy nuclei centred on a shell closure, and for decades the most commonly quoted prediction placed the proton magic number at 114. Popular accounts turned this into the expectation that element 114 itself would be stable, or nearly so.

What was found is more modest and still remarkable. Flerovium-289 lasts about two seconds and flerovium-290 roughly nineteen — enormously longer than the microsecond lifetimes of the neighbouring elements made by other routes, and long enough to do chemistry. But the predicted island is centred on a neutron number around 184, and the isotopes made so far reach only about 175. They sit on the shore, not the summit.

Meanwhile the theoretical prediction itself has moved. Different nuclear models now place the proton shell closure at 114, 120 or 126, and they disagree substantially. That the observed half-lives rise in this region is a measurement; that the rise is caused by a shell closure at Z = 114 specifically is a hypothesis that the models no longer agree on.

The most volatile metal there is

Flerovium is the heaviest element on which any chemistry has been done, and it took three experiments and thirteen years to get a consistent answer.

The question was whether element 114, sitting in group 14 below lead, would behave like a metal at all. Relativistic stabilisation of the 7s and 7p₁/₂ electrons is predicted to be so strong in flerovium that its valence shell is effectively closed, and an early experiment in 2009 was interpreted as showing noble-gas-like behaviour. A second study pointed the other way. The two readings sat unresolved in the literature.

In 2022 a team led by Alexander Yakushev at Darmstadt settled it, using six atoms combined with two from an earlier run. Flerovium adsorbs on gold — so it forms a metallic bond, and is not a noble gas — but it does so far more weakly than mercury does, and considerably more strongly than radon. It also failed to stick to silicon oxide at all. The conclusion is that element 114 is a metal, and the most volatile metal in the periodic table.

That is a measurement, on eight atoms, and it is currently the outer edge of experimental chemistry.