Hassium · element 108
Named after a German state, predicted to be the densest element of all, and it makes a compound just like osmium
Key facts
Hassium takes its name from Hassia, the Latin name for Hesse, the German state where the GSI laboratory in Darmstadt sits. Three atoms were made there in 1984, and for a while element 108 was the heaviest anyone had produced.
If the calculations are right, a lump of hassium would be the densest material in the periodic table, roughly twice as dense as osmium, which holds the record among elements you can actually weigh. Nobody will ever test that claim directly, but chemists have done something almost as remarkable. They have taken a few atoms of hassium, made an oxide from them, and shown that it behaves like osmium tetroxide.
Where it is found
Hassium is not found in nature. There have been occasional claims over the decades that long-lived superheavy nuclei might survive in minerals, but none has stood up to scrutiny, and the accepted position is that every hassium atom in existence was made by people. The total is modest, perhaps a few hundred atoms since 1984, produced in Germany, Russia, Switzerland and Japan.
Two routes are used. The discovery reaction fired iron-58 at lead-208, a cold fusion process giving hassium-265, which lives for about two thousandths of a second, far too brief for chemistry. Instead researchers bombard curium-248 with magnesium-26 to produce hassium-269 and 270, whose half-lives run to several seconds, just enough to carry the atoms out of the target chamber in a gas stream and let them react.
How it was discovered
Peter Armbruster and Gottfried Munzenberg's team at GSI detected three atoms of hassium-265 in March 1984, identifying them by their alpha decay chains to known lighter isotopes. Scientists at Dubna had tried similar reactions around the same time and reported fission events, but fission alone cannot say which element decayed. The joint IUPAC and IUPAP review of the 1990s credited GSI, while acknowledging Dubna's contribution.
Naming was briefly tangled. In 1994 IUPAC suggested hahnium for element 108, recycling the name Americans had used for element 105. The Germans preferred hassium, and in the final 1997 settlement they got their way. It was one of two elements named for GSI's home, the other being darmstadtium for the city itself.
What we use it for
Hassium has no applications. A few atoms that live for seconds cannot be used for anything. Its importance lies in a single, elegant experiment and what it tells us about the periodic table.
In 2001 a team led from GSI and the Paul Scherrer Institute made hassium from curium and magnesium, mixed it with oxygen and watched where the product condensed on a cooled detector. The molecule was hassium tetroxide, HsO4, directly analogous to osmium tetroxide. It condensed at a slightly higher temperature than the osmium version, so it is marginally less volatile, as group 8 trends predict. Physicists also value hassium-270, whose 162 neutrons are predicted to give a small pocket of extra nuclear stability, a preview of the island of stability further out.
Reading the numbers
Handle every figure here with care. The atomic mass of 269 refers to a long-lived isotope, not a natural average. The density of about 41 grams per cubic centimetre is a calculation that depends on hassium adopting a tightly packed crystal structure like osmium. No melting point has ever been measured, and published estimates vary so widely that none is shown here. Boiling point, electronegativity and ionisation energies are unknown.
The configuration [Rn] 5f14 6d6 7s2 places hassium in group 8 with iron, ruthenium and osmium, and predicts a maximum oxidation state of +8. That prediction has been confirmed by the tetroxide experiment, where hassium was in exactly that state. Unlike some of its neighbours, hassium so far shows no obvious relativistic surprises. It is a very heavy osmium, behaving as its position says it should.
Safety and the environment
Hassium is radioactive in every form, with half-lives from milliseconds to, at most, a few seconds for the isotopes made in useful numbers. It cannot persist, cannot accumulate and cannot be transported. The atoms are produced inside shielded accelerator beamlines and are gone almost as soon as they appear. There is no hassium in the environment and no scenario in which the public could be exposed. Its hazard is entirely theoretical.
Did you know?
- Hassium tetroxide, made in 2001 from a handful of atoms, is the heaviest oxide ever detected.
- Predicted at around 41 grams per cubic centimetre, hassium would be nearly twice as dense as osmium and iridium, the densest elements ever weighed.
- Hassium-270 has 162 neutrons, a number nuclear physicists expect to give extra stability to slightly squashed, deformed nuclei.
- The three atoms of the 1984 discovery were hassium-265, with a half-life of around two milliseconds.
- GSI's home state gave hassium its name and GSI's home city gave darmstadtium its name, making Darmstadt the only place with two elements named for its location.
More in the superheavy elements
The nearest neighbours of hassium in the family, by atomic number.
See all 15 superheavy elements on one page, or browse all 118 elements. Also listed under the transition metals.
Frequently asked questions
Why is it called hassium?
The name comes from Hassia, the Latin form of Hesse, the German state where the GSI Helmholtz Centre in Darmstadt is located. The element was first made there in 1984 and the name was approved in 1997.
Is hassium the densest element?
It is predicted to be, at roughly 41 grams per cubic centimetre, but this is a calculation. Only a few hundred atoms have ever been made, so its density has never been measured. Among elements that can be weighed, osmium and iridium hold the record.
What is the half-life of hassium?
The discovery isotope, hassium-265, lasts around two milliseconds. The most useful isotopes for chemistry, hassium-269 and 270, live for several seconds. A few heavier isotopes may live longer, but their half-lives are still being confirmed.
Does hassium form compounds?
Yes. In 2001 researchers made hassium tetroxide, HsO4, from individual atoms and showed it is volatile like osmium tetroxide. It is one of the very few superheavy elements whose compounds have been studied.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Hassium reference article. Spotted an error? Tell us.