It was recalled that dislocations were first invented, circa 1930, in order to explain why pure metal crystals were soft when compared with the homogeneous shear strengths calculated from atomic theory. It was found later that pure semiconductor crystals (e.g., Ge and Si) had room-temperature hardness values which were comparable with calculated homogeneous shear strengths. Furthermore, it was known that pure metal–metalloid crystals (e.g., TiC and WC) were very hard. The differences could not be explained by using a classical mechanical model. However, they could be explained by considering the chemical bonding in differing solid crystals. In particular, hardness depends on the degree of localization of the valence (bonding) electrons. It was noted that selected sets of crystal types should be treated just as selected sets of molecules were treated in theoretical chemistry. Otherwise the rationalization became unmanageable.

Chemical Theory of Dislocation Mobility. J.J.Gilman: Materials Science and Engineering A, 2005, 409[1-2], 7-12