In our previous work, the stopping properties of metallic and covalent films were investigated. Here we consider an ionically bound film. The energy loss of a proton in an LiF monolayer (LiF-1L) is calculated in orbital fashion, based on kinetic theory. The required momentum density and mean excitation energy are obtained from the local density approximation and local plasma approximation respectively. For comparison, the LiF molecule is treated by use of a large intermolecular distance in the film. We find the stopping cross section of the LiF molecule to be only slightly larger than that for the LiF-1L. The Bragg rule (additivity of stopping for the corresponding atoms) is not valid for the ionically bound molecule nor the corresponding extended system, but may be valid if additivity of stopping of atomic ions is assumed. © 1993 John Wiley & Sons, Inc.