A model for determining the energy variations during adsorption of diatomic molecules on metal surfaces is presented. The procedure allows the total bond order to vary based on the probabilities of finding electron pairs in the forming and breaking bonds. Also, the relationship between bond energy and bond order for the gas-phase reactant is described by a Morse potential that is modified to account for experimental bond dissociation energies. With the model, the reaction trajectory and the structure of the corresponding transition state are determined directly from the energy surface. This yields the heat of molecular adsorption and the activation energy for dissociative adsorption and it indicates the conditions that favor dissociative rather than molecular adsorption. The results for adsorption of O 2, N 2, and H 2 on several different metal surfaces show reasonable agreement with available experimental data.