Bond modulus and stability of covalent solids

The chemical stabilities of molecules are determined by their LUMO–HOMO energy gaps. For solids, the analogues of these are their energy band gaps. However, solids are aggregates of molecules, so an energy alone cannot characterize their stabilities. An intensive parameter is needed. Such a parameter is the gap energy per molecular volume. The author has coined the name ‘bond modulus’ for this parameter because it tends to be proportional to elastic moduli and it has the same dimensions. It applies primarily to covalent solids with localized bonding. It is proportional to chemical hardnesses and bulk moduli, as well as octahedral shear moduli and inverse polarizabilities.