The room temperature chemisorption of gaseous NH3 on p-type GaAs(110) has been studied with scanning tunneling microscopy (STM). Atomic-resolution images confirm that the reaction is nondissociative and occurs between lone-pair electrons of the free molecule and cation dangling bonds at the surface. Because the adsorption complex is uncharged, the microscopic dipole screening produced by a single, tilted ammonia molecule is directly observed. The results agree completely with the predictions of Lewis acid-base chemistry.