Rules of chemisorption of atoms and molecules to transition-metal surfaces are analyzed in terms of the distribution of electrons over bonding and antibonding adsorbate-surface orbital fragments. The larger reactivity of low index planes compared to that of the higher index planes is shown to be related to the lower localization energies of valence electrons when the coordination number of surface atoms decreases. For molecular adsorption the relative stability of adsorption complexes is controlled by the balance of donating and backdonating interaction terms.