Scientists have uncovered direct evidence that Einstein’s theory of relativity changes how atoms form triple chemical bonds in heavy elements.
A fundamental rule of chemical bonding begins to break down when atoms become sufficiently heavy. Chemists at Brown University have found direct experimental evidence that Einstein’s theory of relativity changes the structure of triple bonds, challenging a model that has been taught in chemistry classrooms for generations.
The discovery, published in Science, comes from experiments involving carbon and bismuth, a heavy element whose electrons are strongly influenced by relativistic effects. “This idea that relativity is important in heavy elements has been around since the 1970s,” said Lai-Sheng Wang, a Brown chemistry professor and the study’s corresponding author. “But we show direct spectroscopic evidence that what we learned in high school about chemical bonding isn’t true in heavy elements.”
How Relativity Changes Chemical Bonds
Chemical bonds form when atoms share electrons, creating an attraction that holds their positively charged nuclei together. In the conventional description of a triple bond, three shared electron pairs produce one sigma bond and two pi bonds. The sigma bond forms through direct overlap along the axis connecting the nuclei, while the pi bonds involve electron density extending to either side of that axis.
This arrangement accurately describes many bonds involving lighter elements. In heavier atoms, however, the strong positive charge of the nucleus causes some electrons to move at speeds where relativistic effects become significant. These effects strengthen a phenomenon called spin-orbit coupling, which links an electron’s spin to its orbital motion and changes how electrons participate in bonding.
“The boundary between a sigma bond and a pi bond is now sort of smeared,” Wang said. “We still have three bonds, but we don’t really strictly have a sigma or a pi anymore.”
Experiments Reveal a Different Triple Bond
To investigate this effect, Brown doctoral students Deniz Kahraman and Jie Hui, working with Wang and their colleagues, produced molecules containing carbon and bismuth. They cooled the molecules to temperatures near absolute zero before examining them with photoelectron spectroscopy, a technique that uses light to eject electrons and measures their energies to determine how strongly they were bound.
The resulting spectra provided evidence of a triple bond unlike the conventional arrangement. Instead of one sigma bond and two pi bonds, the carbon-bismuth bond exhibited characteristics consistent with one pi bond and two hybrid sigma-pi bonds. This experimentally observed structure supports theoretical predictions that relativistic spin-orbit coupling can fundamentally alter bonding in heavy elements.
Implications for Future Materials
The findings could influence how chemists describe bonds involving heavy elements, particularly as researchers explore their potential in advanced technologies. Bismuth has attracted attention as a possible alternative to toxic lead in next-generation solar cells. It is also being investigated in quantum materials and research relevant to quantum computing.
Understanding how relativistic effects influence its chemical bonds may help scientists develop more accurate descriptions of bismuth-containing compounds and other heavy-element materials. The new evidence could eventually change how these bonds are presented in chemistry textbooks.
“Maybe this will become the new textbook idea as we are dealing with more and more heavy chemistry of the heavy elements,” Wang said.
Reference: “Relativistic collapse of the classical triple bond in the CBi− molecular ion” by Deniz Kahraman, Jie Hui, Xin-Yu Zhang, Neil A. Ellis, Hyun Wook Choi, Kirk A. Peterson and Lai-Sheng Wang, 9 July 2026, Science.
DOI: 10.1126/science.aei1285
The work was funded by the U.S. National Science Foundation (CHE-2403841) and the U.S. Department of Energy (DE-SC0008501).
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