Chemical and physical ”hardness”

Abstract

 A common theme links chemical, mechanical, and optical hardnesses. It is the gap in the bonding energy spectra of materials. This gap determines molecular stability, and therefore ”chemical hardness”. Chemists call it the LUMO-HOMO gap between anti-bonding and bonding molecular orbitals. Physicists call it the band-gap between the conduction and valence energy bands. It forms a basis of various properties; including: chemical reactivity, elastic stiffnesses, plastic flow resistance (dislocation mobility), crystal structure stability, and optical polarizability (refractive indices). These unifying relationships are the subject here. Manipulation of these gaps and their cross-connections plays a critical role in current technology and has led to recent new devices.