When ICI adsorbs on a clean silicon (111)-(7x7) surface, the reaction is chemically selective. This process has been studied with the use of scanning tunneling microscopy and Auger spectroscopy. The dominant mechanism is the formation of silicon monoiodide by abstraction and the ejection of the chlorine atoms back into the gas phase. This pathway is not only chemically selective but also the least exothermic of all the possible reaction mechanisms. Although atomic chemical selectivity in gas-phase reactions is quite common, atomic chemical selectivity is unexpected in gas-surface reactions on clean surfaces because of the high density of reactive sites.