Scanning tunnel microscopy (STM) of Cl/Si(111)7 x 7 at low coverage has been used to carefully measure a trend with sample bias in height contrast between Cl adsorbates and silicon adatoms. The apparent height of the Cl adsorbate increases monotonically with increasing sample bias. In order to help explain the observed trend a model was developed for the tunnel current between a tungsten tip and the Si(111)7 x 7 surface. The model uses a tight-binding approach incorporating known local density of states information calculated using density functional theory. Simulation of the clean surface STM topography yields the correct appearance in both positive and negative sample bias. Application of the model to Cl/Si(111)7 x 7 allowed for a good fit to the observed contrast trend. Valuable new information on the detailed line shape of the Cl-Si sigma* antibonding resonance was obtained. Detailed knowledge of this resonance may well be crucial to future understanding of the unique long-range form of tip-induced diffusion and desorption observed on this system. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3066737]