The reaction mechanism for the dissociative adsorption of molecular chlorine Cl-2 on Si(100) has been studied by density functional theory calculations. The direct activated channel with one end of Cl-2 bonded to a surface Si atom to form a terminal adsorbate dominates the channels studied. The energy barrier for the subsequent complete dissociation from such an adsorbate depends on the adsorption site, and the ordering of the energy barriers can account for the ordering of the statistical weights observed in STM experiments. Our results also fill in the details of the reaction mechanisms as previously probed by a molecular beam surface scattering experiment.