A theory of chemisorption relevant to alkali atoms on metal surfaces is presented. The virtual-impurity-state problem is modeled in the manner presented by Kjöllerström, Scalapino, and Schrieffer for solving the Anderson impurity problem in the low-density approximation (LDA). It is assumed that the binding or chemisorption energy of the alkali metal is composed of two parts, a metallic and an ionic component. Since considerable charge transfer from the alkali atom to the metal occurs, the LDA is appropriate for describing the metallic part of the binding. Another consequence of the large charge transfer is that the major portion of the binding results from an ionic type of bond between the partially charged alkali ion on the surface and a polarization or screening charge inside the metal. The results of the theoretical calculations indicate that binding energies for alkali atoms adsorbed on clean single-crystal faces of metals generally fall with in the range of 1.5-2.5 eV, in accord with available experimental data on metals such as W, Mo, Ta, and Ni. © 1971 The American Physical Society.