Bismuth atoms, positioned near lead on the periodic table, possess significantly heavier nuclei. This increased mass forces their orbiting electrons to move at relativistic speeds — velocities approaching that of light — thereby bringing Einstein’s theory of relativity into play. At these speeds, electrons no longer behave as independent particles with separate spin and orbital motion. Instead, their spin becomes intertwined with their orbit in a phenomenon called spin-orbit coupling, fundamentally altering electron interactions and the nature of chemical bonds.

This nuanced coupling blurs the distinction between sigma and pi bonds, transforming how bonds form and behave. Researchers led by Professor Lai-Sheng Wang conducted experiments on carbon-bismuth molecular ions cooled near absolute zero, analyzing them via advanced photoelectron spectroscopy. This technique knocks electrons from the molecule with laser light and measures their energy to uncover the nature of bonding.

Contrary to traditional models, the spectroscopy data revealed that the bonds in these heavy-element molecules do not conform to one sigma and two pi bonds. Instead, the bonding structure appears as one pi bond combined with two hybrid sigma-pi bonds, a direct signature of relativistic effects modifying chemical bonding. “The boundary between sigma and pi bonds is now smeared,” explains Wang, highlighting a radical departure from classical bonding paradigms.

The implications of this discovery extend beyond academic interest. As other heavy elements gain prominence in materials science—including in quantum computing and solar cell technologies—understanding these relativistic bonding effects could lead to breakthroughs in designing novel materials. Bismuth, already poised as a less toxic alternative to lead, emerges as a compelling candidate in the next generation of quantum materials and sustainable technologies.

Wang and his team believe this spectroscopic evidence represents a pivotal moment for chemists worldwide. The findings challenge longstanding assumptions and may necessitate updates to chemistry curricula and textbooks, acknowledging that the classical dichotomy of sigma and pi bonds does not hold universally, especially under relativistic regimes.

Funded by the U.S. National Science Foundation and Department of Energy, this research bridges atomic physics, quantum mechanics, and chemistry, illustrating a profound example where relativity shapes the microscopic world of chemical bonds.

Subject of Research: Chemical bonding in heavy elements influenced by relativistic effects
Article Title: Relativistic collapse of the classical triple bond in the CBi− molecular ion
News Publication Date: 9-Jul-2026
Web References: http://dx.doi.org/10.1126/science.aei1285

Keywords
Relativistic effects, chemical bonding, bismuth, heavy elements, spin-orbit coupling, photoelectron spectroscopy, quantum materials

For decades, scientists have observed peculiarities in the chemistry of actinides—elements with atomic numbers greater than 88. Compared to their lanthanide neighbors, whose chemistry largely follows well-understood patterns driven by the filling of 4f orbitals, actinides introduce complexities influenced heavily by relativistic interactions among their 5f electrons. These effects become even more pronounced as we climb the periodic ladder towards superheavy elements with atomic numbers equal to or exceeding 104, where nuclear charge smashes electron speeds into a realm where special relativity must be seriously accounted for.

Experimental insights into these exotic elements, however, remain frustratingly sparse. Once we move beyond fermium (Z=100), the production of atoms becomes incredibly challenging, necessitating facilities capable of synthesizing these fleeting species in minuscule quantities—sometimes only one atom at a time. These atoms decay extraordinarily quickly, eliminating conventional bulk-chemistry assessments and demanding cutting-edge experimental strategies to glimpse their behaviors.

In a groundbreaking study conducted at the Lawrence Berkeley National Laboratory, scientists have reported a pioneering approach that directly identifies molecular species formed by heavy elements on an atom-by-atom basis. Using the 88-Inch Cyclotron facility, nuclei of actinium (Ac, Z=89) and nobelium (No, Z=102) were produced through nuclear reactions and immediately exposed to trace amounts of reactive gases—water vapor (H₂O) and nitrogen (N₂). This exposure allowed the atoms to form compounds whose existence was subsequently confirmed by measuring their mass-to-charge (m/z) ratios.

Central to this breakthrough was the utilization of the FIONA (For the Identification Of Nuclide A) technique. FIONA provides exceptionally precise mass spectrometric analyses optimized for identifying individual atoms and their associated molecules, even when only a handful of species are generated. By capturing these rare species and recording their m/z signatures, the researchers directly pinpointed molecules containing Ac and No atoms, marking the first such direct identification of heavy-element molecular species employing an atom-at-a-time methodology.

This remarkable capability opens an entirely new frontier in superheavy-element chemistry. Given the scarcity and transient existence of these atoms, observing molecular formation directly grants chemists leverage to probe their bonding preferences, reaction pathways, and electronic structures—parameters that are often mired in theoretical ambiguity due to intense relativistic effects and limited experimental data.

The relativistic contraction and expansion of atomic orbitals in superheavy elements have been predicted to dramatically influence chemical affinities and oxidation states. For instance, the 7s and 7p orbitals may become more contracted and stabilized, whereas 6d and 5f orbitals could expand or destabilize differently, upsetting conventional orderings. Confirming these theoretical predictions with empirical data has been an elusive goal, primarily because the fleeting elements decaying in milliseconds or less leave no room for traditional chemical analysis.

By capturing and identifying molecules involving Ac and No under ultra-dilute gas conditions, researchers have not only validated hypothesized reaction pathways but also established the feasibility of employing atom-at-a-time techniques to study superheavy-element chemistry. This suggests a promising route forward to explore the chemistries of other barely tangible elements at the periodic table’s frontier, such as lawrencium (Lr) and elements beyond.

Understanding the bonding and reactivity of these heavy elements informs fundamental questions extending beyond pure chemistry. Their behaviors influence nuclear stability, help refine theoretical models including quantum electrodynamics corrections, and potentially impact the search for new, stable isotopes or superheavy “islands of stability.” Moreover, unlocking accurate elemental properties facilitates material science advancements when such atoms occur even transiently in nuclear reactors or astrophysical events.

The methodology employed in this study also represents a triumph of experimental ingenuity. Engineering a system to combine nuclear synthesis, immediate chemical reaction with trace gases, and mass spectrometric detection with unparalleled sensitivity exemplifies how multidisciplinary approaches can surmount formidable challenges in elemental science. It exemplifies the synergy between nuclear physics, analytical chemistry, and computational modeling.

This interdisciplinary success could catalyze a paradigm shift, steering efforts from indirect detection methods—such as decay chain identification—to direct molecular characterization of the most elusive members of the periodic table. Such transition empowers chemists to derive unambiguous chemical signatures, identify molecular geometries, and test relativistic quantum chemistry predictions with unprecedented fidelity.

Ultimately, this work shines a light on the limits and possibilities inherent at the table’s end. While relativistic effects complicate the classical periodic trends, they do not render the heaviest elements’ chemistry inscrutable. Instead, new experimental avenues like the atom-at-a-time molecular identification herald a refined understanding of how matter behaves under the most extreme atomic conditions. The periodic table may not vanish at its lower reaches, but rather transform into a more intricate and fascinating landscape molded by relativistic quantum phenomena.

As research continues to push the boundaries of nuclear synthesis and chemical detection, the convergence of theory and experiment promises to reveal a landscape of elemental chemistry richer and more complex than ever imagined. The identification of actinium and nobelium molecular species thus stands as a cornerstone achievement, assuring the future exploration of superheavy chemistry will be rooted in rigorous experimental foundation, informing not only the periodic table but the deep nature of matter itself.

Subject of Research: Direct identification and chemical characterization of molecules containing actinide (Ac) and superheavy (No) elements using atom-at-a-time experimental techniques.

Article Title: Direct identification of Ac and No molecules with an atom-at-a-time technique.

Article References:
Pore, J.L., Gates, J.M., Dixon, D.A. et al. Direct identification of Ac and No molecules with an atom-at-a-time technique. Nature (2025). https://doi.org/10.1038/s41586-025-09342-y