On a Si(100)-(2x1) surface with abundant dangling bonds, reaction of HCl molecules at room temperature is dominated by exothermic dissociative adsorption of H and Cl on two adjacent dangling bonds. This co-adsorption reaction is blocked for an isolated dangling bond, yet surprisingly endothermic H or Cl abstractive adsorption occurs, as observed by in situ scanning tunneling microscopy. On an isolated dimer dangling bond pair, co-adsorption of H and Cl is common as expected, but adsorption of a pair of abstracted Cl or H from two HCl molecules also occurs. These results, complemented by theoretical calculations, indicate that dissociative adsorption and abstractive reaction of a multi-atom gas molecule can be initiated at a single DB by forming an intermediate adsorption state.