Small finite models of one- and two-dimensional crystals are shown to predict the same localized states (i.e., surface states) as semi-infinite models. An SCF—LCAO—MO calculation for a 6-atom linear chain where each atom has one valence electron of s-type indicates that simple Hückel models give a reasonably good description of the orbital changes that occur as the electron-attracting power (i.e., electronegativity) of an end atom is changed, to simulate chemical adsorption. The utility of small finite models is demonstrated in calculations of the effect of surface geometry on adsorption. Adsorption bond orders are greater for adsorption on a corner site than on a middle edge-atom. A similar result is found by using free valences. With adsorption bond orders it is essential to consider not only the contribution from any localized state, but also the contributions from all the occupied molecular orbitals.