Elucidation of bonding trends from variability in Atomic Solid State Energies

Abstract The solid state energy (SSE) of an element in a specified oxidation state corresponds to the average energy of a frontier orbital, i.e., the energy of the lowest-unoccupied state (electron affinity) for a cation or the highest-occupied state (ionization potential) for an anion. Thus SSE, like electronegativity, accounts for diverse chemical properties by assigning a single, scalar quantity to an element. SSEs are evaluated by averaging electron affinities (EA) or ionization potentials (IP) for all relevant compounds within the database. For a given element, EAs or IPs cover a range of energies; we define this distribution to be SSE variability. We show that SSE variability reveals a systematic chemical trend: as interatomic distances decrease, EA moves upward toward the vacuum level, while IP moves downward. Consequently, the band gap increases around a universal energy reference at −4.5 eV. In an elemental solid, this relationship between EA/IP energy and interatomic distance arises from electronic-charge sharing (covalent bonding), while in a binary compound it arises from electronic charge redistribution (polar covalence). By extension, we suggest that this EA/IP-interatomic distance trend does not arise from electronic charge transfer (ionic bonding).