Hassium (Hs)

Osmium tetroxide is a familiar and unpleasant substance: volatile, pungent, used to stain biological specimens. In 2002 a group of chemists made the same compound out of element 108, sent it down a cooled channel lined with detectors, and watched where it condensed. It condensed almost where osmium tetroxide does. Element 108 is a group 8 metal, and that conclusion rests on fewer than ten atoms.

A tetroxide, near enough to osmium's

The experiment, led by Christoph Düllmann and published in Nature, worked by turning a notoriously difficult problem into a temperature reading. Hassium atoms produced in an accelerator were swept into a stream of oxygen-bearing gas, where anything behaving like a group 8 element would form the tetroxide. That gas then passed along a narrow channel between two facing rows of alpha detectors, with a temperature gradient running from around −26 °C at the entrance down to about −97 °C at the far end.

A volatile compound travels until it is cold enough to stick. Where each hassium atom stuck was recorded by the detector it decayed on top of, and comparison with osmium tetroxide run through the same apparatus gave a direct measure of how the two compounds differ.

They differ slightly. Hassium tetroxide is a little less volatile than osmium's, deposited at a marginally higher temperature. That is a small, specific, quantitative result, and it is what "element 108 is a group 8 metal" actually means in practice.

Later work extended it: hassium tetroxide reacts with a sodium hydroxide surface to form a hassate, the same acid-base behaviour osmium shows, which pushed element 108's characterisation one step beyond mere volatility.

Hassia

The element was made in 1984 at the heavy-ion laboratory outside Darmstadt by Gottfried Münzenberg's group, from a lead target under an iron beam, with three atoms detected through their alpha decay chains. Dubna had reported earlier attempts that were not accepted, and the Transfermium Working Group credited Darmstadt.

The name is the Latin for Hesse, Hassia — the German federal state in which the laboratory sits. It is the only element named after a German state, and it belongs to a small cluster of names in this part of the table that commemorate geography rather than people. The discoverers' habit of naming for their own surroundings has left elements 105 through 112 reading, in places, like a gazetteer.

The island that is not round

Element 108 contributes something specific to the theory of why any of these nuclei survive at all.

Nuclear stability comes in part from shell closures — particular numbers of protons or neutrons that pack into complete shells and bind unusually tightly, the nuclear equivalent of a noble gas configuration. The famous prediction is a spherical closure far up the chart, but calculations also predict closures for nuclei that are not spherical at all, in particular a deformed shell gap at 108 protons and 162 neutrons. A nucleus with those numbers would be stabilised while being distinctly rugby-ball shaped.

Hassium-270 has exactly that composition. It was produced in 2004 by a group led by Jan Dvorak, and its half-life came out around ten seconds — long, in this neighbourhood, and longer than the neighbouring isotopes. That is consistent with the predicted deformed shell closure and is generally treated as the best experimental support for it.

The distinction matters. The shell gap at 108 and 162 is a theoretical prediction from nuclear models; the half-life of hassium-270 is a measurement. The measurement fits the prediction, which is evidence, not proof, and the models that generate it disagree with each other elsewhere on the chart.

Searches in osmium ore

Because hassium sits directly below osmium, and because a sufficiently long-lived isotope might in principle still be around from the birth of the solar system, several groups have looked for naturally occurring hassium in osmium-bearing minerals. Claims of a positive detection have been published and have not been accepted; no confirmed natural hassium exists. The searches were not unreasonable — they test the same shell-stabilisation ideas from the opposite direction — but the current position is that every hassium atom that has ever been observed was made deliberately.