An intersecting-state model has been applied to the calculation of activation energies in elementary gas-phase reactions. For thermoneutral reactions the sum of the bond distensions, d, at the transition state is found to depend on the sum of the equilibrium lengths of the reactive bonds and on the bond order at the transition state. For non-thermoneutral reactions d is found to have a quadratic dependence on the enthalpy of the reaction. Such dependence is affected by the so-called configuration entropy. A hierarchy of electronic factors for chemical reactions has been found and associated with the overall bond order of the activated complexes, which is considered to vary over the reaction path. Transition-state bond orders higher than those of the reactants or products are interpreted in terms of the rearrangement of a pair of electrons from occupied antibonding or non-bonding orbitals which acquire a bonding character at the transition state. The model appears to be general and encompasses earlier models relating thermodynamic and kinetic parameters of chemical reactions.