Core Electron Bonding at Low Pressure: A New View of Chemical Bonds
It’s long been taught that core electrons—the deeply buried electrons closest to the nucleus—don’t take part in chemical bonding. They’re considered too tightly held to be influenced by neighboring atoms’ electrons, leaving the outer, or valence, electrons to do the bonding work. Yet a new line of quantum chemical calculations from researchers at the University at Buffalo is challenging this conventional wisdom. The team suggests that, for some elements, core or semicore electrons can participate in bonding at pressures far below what was once believed, and in some cases even at ambient atmospheric pressure.
The study, described in the Journal of the American Chemical Society, focuses on the semicore electrons of alkali metals—the first-row, highly reactive elements famous for their simple electron configurations. The researchers found that these semicore electrons can engage in bonding at pressures of a few gigapascals, which occur in the deep Earth’s crust and upper mantle, but are well below the hundreds of gigapascals previously thought necessary for core-electron participation. In cesium, the team found evidence that semicore electrons may bond even at ambient pressures, a finding with significant implications for how we understand bonding in common elements.
What the Findings Mean for Bonding Theory
Traditionally, core electrons were assumed to be spectators in chemical reactions. The UB work reveals that, under the right conditions, semicore electrons can become active participants in bonding, helping to drive and stabilize particular crystal structures. In the case of cesium chloride, the researchers highlight the B1–B2 transition—where a material’s crystal geometry shifts from a more octahedral arrangement to a more cubic one. The involvement of semicore electrons in bonding appears to be a key factor in this transition, suggesting that the energy landscape governing phase changes under pressure is more nuanced than previously understood.
How the Researchers Reached These Insights
The team, led by Eva Zurek, PhD, SUNY Distinguished Professor in UB’s Department of Chemistry, and Stefano Racioppi, PhD, used advanced quantum chemical calculations to model the behavior of semicore electrons in alkali metals as they bond with fluorine and experience pressure-induced transitions. Instead of solving the full, intractable many-electron Schrödinger equation, they employed sophisticated approximations and models run on UB’s Center for Computational Research. Their results indicate that core-electron contributions to bonding are not as rare or as pressure-demanding as once thought.
Why This Matters for Earth and Planetary Science
The implications extend beyond pure chemistry. If semicore electrons can bond under relatively modest pressures, our models of how elements behave deep inside planets—and how planets form and evolve—could change. Bonding patterns influence everything from a planet’s radius and interior dynamics to tectonic activity and magnetic field generation. In other words, a subtle revision of core-electron participation could ripple through planetary science and astrobiology, altering estimates of which worlds might support life and how their geologies evolve over time.
Next Steps: From Theory to Experiment
While the current work is theoretical, the authors outline concrete experimental directions. They suggest employing X-ray diffraction to characterize the atomic structures of alkali metals under pressure and to probe the role of semicore electrons in bonding more directly. As Zurek notes, this research should serve as a roadmap for experimentalists to test whether core-electron bonding can be observed at lower pressures than previously assumed. If validated, these findings could recalibrate our understanding of chemical bonding and the behavior of materials under extreme conditions.
Bottom Line
Core electron bonding was once thought to require extreme pressures deep inside planets. The UB study shows that semicore electrons in alkali metals can participate in bonding at surprisingly low pressures—sometimes even at ambient conditions. This challenges established chemistry paradigms and opens new avenues for studying how elements bond, how planets form, and how we model the interior dynamics of Earth-like worlds.
