CHEMICAL AND PHYSICAL HARDNESS

A common theme links chemical, mechanical, and optical hardnesses. It is the gap in the bonding energy spectra of materials. This gap determ ines molecular stability, and therefore ''chemical hardness''. Chemist s 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; includin g: chemical reactivity, elastic stiffnesses, plastic flow resistance ( dislocation mobility), crystal structure stability, and optical polari zability (refractive indices). These unifying relationships are the su bject here. Manipulation of these gaps and their cross-connections pla ys a critical role in current technology and has led to recent new dev ices.