Core Electron Bonding Reexamined
Researchers from the University at Buffalo have challenged a long‑standing tenet of chemistry: core electrons, nestled deep inside an atom near the nucleus, are typically considered nonparticipatory in bonding. In a study published in the Journal of the American Chemical Society, UB scientists used advanced quantum chemical calculations to show that the semicore electrons of alkali metals can engage in bonding with fluorine under surprisingly modest pressures. In fact, for cesium, semicore bonding may occur at ambient pressure—roughly a million times lower than the pressures found deep inside the Earth.
What the study reveals about semicore electrons
The team focused on alkali metals—the highly reactive elements occupying the first row of the periodic table—and investigated how their semicore electrons participate in chemical bonding. The findings indicate that semicore electrons can contribute to bonds and influence structural changes in compounds, particularly during a pressure‑induced B1–B2 transition. This transition describes a rearrangement of the atomic crystal structure from the octahedral NaCl‑type arrangement (B1) to the cubic CsCl‑type arrangement (B2). The results suggest that core or semicore electrons are not always spectators under pressure, but can be active players in determining molecular geometry and stability.
Pressure thresholds and the B1–B2 transition
One striking outcome is that semicore bonding requires only a small fraction of the pressures once thought necessary. The calculations show that alkali metals’ semicore electrons can participate in bonding at just a few gigapascals, pressures typical of the deep crust and upper mantle. Even more notable, cesium’s semicore electrons appear capable of bonding at ambient pressure, a stark contrast to prior expectations and highlighting the potential for semicore activity under everyday conditions.
How the researchers approached the problem
Explaining electron behavior in real materials is an enormous theoretical challenge because the Schrödinger equation for many interacting electrons is computationally intractable in its exact form. UB and collaborators turned to quantum chemical calculations and sophisticated modeling to render the problem solvable. The work, supported by UB’s Center for Computational Research and the Center for Matter at Atomic Pressure, examined how semicore electrons in alkali metals interact with fluorine while the material undergoes the B1–B2 transition. The study’s co‑corresponding authors— Eva Zurek, SUNY Distinguished Professor in UB’s Chemistry Department, and Stefano Racioppi, now at the University of Cambridge—emphasized that semicore bonding can drive and stabilize the cubic B2 phase observed in cesium chloride under certain conditions.
Why this matters for planetary science and materials chemistry
The implications extend beyond fundamental chemistry. If electrons bond differently under pressure than previously assumed, it could alter our models of how elements behave deep inside Earth and Earth‑like planets. Changes in bonding could influence a planet’s radius, tectonic dynamics, and magnetic field generation—factors that ultimately bear on a planet’s ability to support life. The findings provide a conceptual roadmap for future experiments and offer a way to reinterpret how planetary interiors evolve under extreme conditions.
Next steps: experimental validation
The authors acknowledge that this is not purely a theoretical exercise. They propose experimental pathways, such as using X‑ray diffraction to characterize the atomic structure of alkali metal compounds under varying pressures to confirm the role of semicore electrons in bonding and the B1–B2 transition. As Zurek notes, the study could serve as a practical guide for experimentalists aiming to verify the predictions of semicore‑electron bonding at pressures closer to ambient conditions than previously assumed.
Bottom line
By suggesting that core, semicore electrons can participate in chemical bonding at comparatively low pressures, this research challenges classic chemistry dogma and opens new avenues for understanding matter under pressure. The findings have implications for chemistry, materials science, and planetary evolution, potentially reshaping how we model the behavior of atoms when nature presses in from all sides.
