An international team has measured the strength of a chemical bond in a superheavy element for the first time, and the seaborgium bond is weaker than the periodic table says it should be. The result was published in Nature and announced by the University of Jyväskylä on Oct. 9.
What the Team Measured
Seaborgium is element 106. It does not occur in nature. Laboratories make it one atom at a time, and the atoms last only fractions of a second. The researchers worked at the GSI accelerator laboratory in Darmstadt, Germany, using a separator called TASCA, according to the University of Jyväskylä release.
The team built a molecule called seaborgium hexacarbonyl, in which one seaborgium atom binds six carbon monoxide molecules. Chemists write it as Sg(CO)6. The molecule had been made once before, about 10 years ago at the RIKEN Nishina Center in Japan, but no one had managed to measure how tightly it holds together.
The team compared seaborgium with tungsten, which sits directly above it in the periodic table and forms the matching compound W(CO)6 easily. The paper, titled “Observation of relativistic bond weakening in seaborgium hexacarbonyl”, gives the first bond dissociation energy of Sg(CO)6 as about 4 kilojoules per mole lower than that of tungsten’s version, at roughly 188 kilojoules per mole.
Why the Result Surprised Chemists
The chromium group of elements, which runs chromium, molybdenum, tungsten and then seaborgium, has a pattern. The hexacarbonyl bond gets stronger as the elements get heavier. Juha Uusitalo, a senior researcher at Jyväskylä who took part in the experiment, said in the university release that chemists expected the bond to keep strengthening from chromium through seaborgium.
It did the opposite. Seaborgium broke the trend, and the team attributes the break to relativistic effects. In atoms this heavy, the inner electrons move at a large fraction of the speed of light, which changes the shape and energy of the outer orbitals that do the bonding. The paper’s abstract says the result matches theoretical predictions that relativity destabilizes the seaborgium 6d orbitals, the electron shell that does much of the bonding.
The prediction was not perfect. The measured drop is smaller than some calculations expected. The abstract says the measurement agrees with theory in direction, so theorists now have a measured number to test their calculations against. The release says experiments with elements heavier than seaborgium are next, and each new measurement gives the calculations a fresh check.
Three of 75 Atoms Formed the Molecule
The release gives the count behind the headline. The team registered 75 seaborgium atoms in total, and three of them showed up in the hexacarbonyl detector. For tungsten, 17% of atoms formed the matching compound.
The arithmetic shows how a small energy gap shows up in a data set this thin. Three of 75 is 4%. That is about one quarter of the tungsten rate. A bond only 4 kilojoules per mole weaker, roughly 2% of the total bond energy, produced a yield roughly four times lower. The experiment works as a magnifier: the chemistry is so sensitive to bond strength that a few kilojoules decide whether a molecule survives the trip to the detector.
The abstract reports a second number from the same run. Sg(CO)6 clings to a gold surface with an adsorption enthalpy of roughly 45 kilojoules per mole, according to the repository record. That figure describes how the intact molecule sticks to the detector, and it is separate from the bond inside the molecule. Together the two values describe a molecule that forms rarely and holds together a little less firmly than its tungsten cousin.
The method has a cost. With only three detections, one fewer or one more changes the yield by a third. The release does not state an error bar for the yield, and the paper’s abstract, as listed in the Jyväskylä repository, gives the bond result as an approximate figure. Readers should treat the 4 kilojoules as a first measurement, not a final one.
Why a Few Atoms Matter
Superheavy elements are those with atomic numbers 104 to 118. According to the release, laboratories make at most a few atoms of them per day, and they live seconds at most. Chemistry on single atoms has to be done on the fly, with the separator pulling the atom out of the beam, sending it through a gas stream, and catching it on a detector before it decays.
Uusitalo said the work shows what long-term international collaboration can achieve. He has taken part in TASCA experiments nearly every year since the separator was finished about 20 years ago, and Jyväskylä has been a partner since the start.
The practical point is that the periodic table is a map drawn from lighter elements. Chemists use it to guess what an unmade compound will do. At seaborgium the guess failed in a measurable way, which means the same guess for elements 107 and above may fail too. The release says the group is now preparing experiments with elements heavier than seaborgium.
The Jyväskylä repository record lists 24 authors on the paper, with Uusitalo among them, and places it in volume 658 of Nature, pages 337 to 341. A. Yakushev is listed first on the byline.
