In the last chapter, in the section on molecular theories of fracture, almost throughout an Arrhenius equation has been used to describe the activation of element breakage. The energy of activation, U0, frequently turned out to be equal to (or was assumed to be equal to) the dissociation energy of the weakest main chain bond. Before further analyzing the kinetics of element — and possibly chain — breakage a definition of the mechanical strength of a bond and of a chain have to be given. In order to do this basic results of quantum chemistry [1,2] are recalled in this chapter which concern the “strengths” of intramolecular bonds and factors influencing the binding potential such as electronic excitation or ionization.