Relativity changes the textbook chemistry of heavy-element bonds

Relativity strengthens spin-orbit coupling. This coupling links an electron’s spin to its orbital motion around the nucleus and changes how the electron takes part 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.”

The carbon–bismuth bond therefore appears to contain one pi bond and two hybrid sigma–pi bonds, rather than one pure sigma bond and two pure pi bonds.

Takeaway: Relativity blurs the line between the two familiar ways electrons form a bond.

How did the team test the prediction?

Brown doctoral students Deniz Kahraman and Jie Hui, working with Wang and their colleagues, produced molecules containing carbon and bismuth. The team then cooled those molecules to temperatures near absolute zero, the lowest possible temperatures.

The researchers examined the molecules with photoelectron spectroscopy. This technique uses light to eject electrons from a sample and measures their energies, revealing how strongly those electrons were bound.

The resulting spectra showed features consistent with one pi bond and two hybrid sigma–pi bonds. Here, “hybrid” means that the usual sigma and pi characteristics mix rather than remaining sharply separated.

This experimentally observed structure supports theoretical predictions that relativistic spin-orbit coupling can fundamentally alter bonding in heavy elements.

Takeaway: The measured electron spectrum matched a new bonding pattern rather than the classroom model.

What changes for chemists?

The result may influence how chemists describe bonds involving heavy elements, particularly as they explore compounds for advanced technologies. For practicing chemists, the immediate change is interpretive: assigning bond types from spectra can no longer rely on a rigid one-sigma, two-pi template for sufficiently heavy atoms.

Bismuth has attracted attention as a possible alternative to toxic lead in next-generation solar cells. Researchers also study it in quantum materials and in work relevant to quantum computing.

Understanding the relativistic effect may help scientists develop more accurate descriptions of bismuth-containing compounds and other heavy-element materials. Wang suggested the finding may become the new textbook idea as heavy-element chemistry becomes more prominent.

Takeaway: The immediate payoff is a more accurate account of heavy-element bonds, while practical uses remain under investigation.

What did the study establish?

The reported evidence centers on the carbon–bismuth system and its molecular spectra, rather than device performance. The findings support an altered bond pattern and the predicted mechanism in that system; applications in solar cells, quantum materials and quantum computing remain prospective.

The paper, “Relativistic collapse of the classical triple bond in the CBi− molecular ion,” appeared in Science on July 9, 2026. A molecular ion is a charged molecule, and the minus sign identifies this species as negatively charged. Deniz Kahraman, Jie Hui, Xin-Yu Zhang, Neil A. Ellis, Hyun Wook Choi, Kirk A. Peterson and Lai-Sheng Wang authored the study; its DOI is 10.1126/science.aei1285.

Funding came from the U.S. National Science Foundation through grant CHE-2403841 and the U.S. Department of Energy through grant DE-SC0008501.

Takeaway: Heavy-element bonding needs a physics model that explicitly includes relativity.