At the Edge of the Periodic Table, Relativity Breaks a Chemical Trend

In October 2026, a team of chemists and physicists at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, published the first direct measurement of a chemical bond in a compound of a superheavy element. Their subject was seaborgium, element 106, a metal that does not occur in nature and that the team had to manufacture one atom at a time inside a particle accelerator. The result, reported in the journal Nature, is a small number with large implications: the bond between a seaborgium atom and a carbon monoxide ligand is weaker, by about 4 kilojoules per mole, than the same bond in tungsten, seaborgium's lighter periodic-table sibling. The periodic trend said the bond should keep getting stronger. Relativity said otherwise, and relativity won.

Chemistry at the edge of the periodic table

Superheavy elements, those with atomic numbers from 104 to 118, are made by fusing lighter nuclei together at accelerator facilities, and they arrive at rates of at most a few atoms per day. Their lifetimes are measured in seconds or fractions of a second, which makes chemistry, the science of how atoms share and trade electrons, extraordinarily difficult to do. You cannot fill a beaker with seaborgium. You cannot weigh it, titrate it, or crystallize it. Every molecule you make contains exactly one atom of the element, and that atom announces itself only by the radioactive decay it undergoes before vanishing.

The reason anyone goes to this trouble is that the periodic table is, at its far edge, a prediction about relativity. In heavy atoms, electrons near the nucleus are pulled so hard that they reach a substantial fraction of the speed of light, and special relativity makes them effectively heavier. The heavier electrons sit closer to the nucleus and shield its charge better, pushing the outer electrons, the ones that do chemistry, farther out. The net effect is that a superheavy element can behave very differently from what its column in the periodic table would suggest.

Seaborgium sits in group 6, below chromium, molybdenum, and tungsten. In their hexacarbonyl complexes, molecules of the form M(CO)6, the metal-carbon bond strength increases steadily down the column, and extrapolation said seaborgium should continue the trend. Relativistic quantum-chemical calculations said the trend would reverse; calculations without relativity kept predicting the rise. For years no measurement could settle the argument, because nobody could catch enough seaborgium chemistry to measure anything at all.

One atom at a time

The molecule at the center of the experiment, seaborgium hexacarbonyl or Sg(CO)6, was first synthesized about a decade ago at the RIKEN Nishina Center in Japan by researchers from Johannes Gutenberg University Mainz and GSI. Its bonding was predicted to be unusually sensitive to relativistic effects, making it the ideal test case. But the first attempts to extract a bond-strength number from the handful of available atoms did not succeed.

The 2026 experiment used TASCA, the TransActinide Separator and Chemistry Apparatus, at GSI. The team produced the isotope seaborgium-259, whose two nuclear states live for less than half a second. Each atom was separated from the fusion debris and brought to rest in a helium and carbon monoxide gas mixture, where the chemistry happened on the fly. Atoms that formed a volatile hexacarbonyl sailed past a first detector array, which caught all non-volatile fragments, and reached a refrigerated second array that adsorbed the volatile complexes. Each atom was identified by its radioactive decay signature. Because the fraction of atoms completing all six additions is governed by the energy of the final bond, counting atoms is, in effect, measuring chemistry.

The team ran the same setup with tungsten, using the short-lived isotopes tungsten-161 and 162 for a fair comparison. Of 75 registered seaborgium atoms, 3 reached the second detector as Sg(CO)6, a yield of about 4 percent, versus 17 percent for tungsten. From that gap, the researchers deduced that the sixth metal-carbon bond in the seaborgium complex is weaker by 4 kilojoules per mole, plus or minus 2. The smooth climb in bond strength from chromium through molybdenum to tungsten turns around at seaborgium, exactly as the relativistic calculations predicted.

Seventy-five atoms of seaborgium were made and caught, one at a time, and three of them completed the molecule. From that count came the first measured bond energy in superheavy chemistry, and a clean verdict: relativity reshapes the valence shell at the edge of the periodic table.

Why the weakening happens

The explanation lives in seaborgium's 6d orbitals, the valence orbitals that donate and receive electrons in the metal-carbon bonds. In a carbonyl complex, bonding runs in two directions at once: the carbon monoxide ligand donates electron density to the metal (sigma donation), and the metal pushes density back into the ligand's empty orbitals (pi backbonding). Relativity destabilizes seaborgium's 6d orbitals, and the calculations show that both the sigma donation and the pi backbonding are reduced in Sg(CO)6 relative to its tungsten analog. Each channel weakens, and the measured bond energy drops.

This is a direct experimental confirmation of machinery chemists rely on far beyond the superheavy world. Relativistic quantum-chemical methods are used in computational materials design, catalyst screening, and heavy-element modeling from nuclear waste handling to next-generation electronics. They were trusted because they reproduce ordinary chemistry, but their predictions at the extremes were long unverified by direct measurement. The seaborgium result is a calibration point at the far edge, won with the same methods now running on supercomputers to design the technologies of the next decade.

What comes next

The experiment also matters as pure instrumentation. Doing gas-phase chemistry on atoms that live less than half a second, separating them from accelerator debris, and identifying individual molecules by their decay radiation is an analytical tour de force, opening a path to compounds of elements even heavier than seaborgium. The collaboration, led by GSI with the Helmholtz Institute Mainz and partners in Prague, Liverpool, Roorkee, Tokai, and Jyvaskyla, is already preparing experiments beyond seaborgium.

There is a quiet elegance to the enterprise. The periodic table has survived more than 150 years by predicting the behavior of atoms nobody had made yet. At its frontier, its predictions are not wrong so much as incomplete: they describe the chemistry that would exist if electrons were slow. They are not. The fastest-moving things most of us will never see are inside the atoms of the heaviest elements, and their speed is now measurable, four kilojoules at a time, in bonds that last a fraction of a second and then are gone.