The observation method for variable bond orders and atomic valences on a reaction path is the valuable practical application of the DF Connectivity Matrix. 52,63, We have proved that the derivatives of the DF connectivity matrix elements over the reaction progress parameters (Reaction Fragilities) serve as fine probes of the electron density evolution upon a This observation is focused directly on the valence region of a reacting molecule, as it has been noticed by Salem, that core electron region of the spherical symmetry does not contribute to the electronic H-F force acting on a nucleus. The energy analysis presented hereby provided the justification for the Reaction Fragility Method: the bond fragilities represent a measure for the anharmonicity of bonds and contacts in the actual configuration of the nuclei at the corresponding stationary state of the electron density. In the chemical language, anharmonicity can be translated as the susceptibility of a bond to changing its strength, the formidable point of view for a chemist interested in transformations of molecules -chemical reactions.