Energies of activation are calculated for hydrogen abstractions by radicals, using bonding and antibonding Morse curves. The model is not parametric. Forty seven reactions are treated, including many involving abstractions from silicon, sulfur, germanium, and tin hydrides. The overall average deviation between experimental and calculated values is less than 1 kcal/mol. For X-H + Y-. to give X(.) + H-Y, the calculation requires the following input data for X-H, H-Y, and X-Y : bond dissociation energy, bond length, and infrared stretching frequency. The model indicates that the properties of the X-Y bond have a major effect on the energy of activation. The importance of repulsive forces is highlighted. Several puzzling patterns of known reactivities are explained satisfactorily. From the rules established, reasonable predictions can be made without actually carrying out the calculation.