The reaction force F(Rc) of a process is the negative derivative of the potential energy V(Rc) of the system with respect to the intrinsic reaction coordinate Rc. For the dissociation of a bond, F(Rc) has a negative minimum at a point Rc = α, while for the formation of the same bond, F(Rc) has a maximum at Rc = α. These extrema divide the respective processes into two phases. For the dissociation, these correspond to an initial stretching of the XY bond (structural effect) followed by separation into the fragments X and Y (electronic effects); for bond formation, the electronic phase is first (attractive interaction and gradual bond formation) and then the structural (relaxation to equilibrium separation). The noteworthy feature is that the key point Rc = α always is such that V(α), relative to the minimum of V(Rc), is about 27% of the dissociation energy. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007