To arrive at a clear and atomically realistic representation of the process of ionic screening, a model with the following necessary and justifiable constraints was devised. 1) The minimum internuclear distance (IND) between a negative site on the membrane and a cation screening the site is equal to the sum of the site's "equivalent" radius (rs) + the diameter of a water molecule (approximately 2.8 A) + crystal radius of the cation (rc). 2) The average value for the dielectric constant (D) over IND is given by D approximately 80 ((IND - rs - rc)/IND). When this simple atomic model for ionic screening is employed in conjunction with equilibrium ion-selectivity theory, it is possible to predict quantitatively, from coulombic energy calculations, the secondary stereospecific actions of certain alkaline-earth cations as well as the predominant screening effect of these divalent cations at the surfaces of different types of membranes. The model also successfully predicts the transition from a predominantly screening situation to a predominantly binding situation, which was observed experimentally when negative surface charge density was decreased in nerve.