The adsorption of monoenergetic IBr molecules on the Si(111)-7X7 surface has been studied using scanning tunneling microscopy, mass spectrometry, Auger electron spectroscopy, and supersonic molecular beam techniques. The adsorption proceeds predominantly via the direct abstractive adsorption mechanism and preferentially occurs at the center Si adatoms. The IBr abstraction probabilities at the incident energies of 0.15 and 0.82 eV have been determined to be 0.90+/-0.03 and 0.77+/-0.03, respectively. The minor dissociative adsorption channel of IBr can be enhanced at the expense of the abstractive adsorption channels by raising the incident energy. Most importantly, no atomic selectivity for iodine or bromine was observed. A reaction mechanism involving two types of transition states, Si ... I ... Br-(s) and Si ... Br ... I-(s), has been proposed to interpret the experimental observations. The attractive interaction between the nearly symmetric highest occupied molecular orbitals (HOMO, pi* antibond) of IBr and the partially-filled Si adatom dangling bonds governs the surface site selectivity and the atomic selectivity of IBr adsorption on Si(lll). Comparison with the adsorption of ICl on the surface has also been made to clarify the role of the asymmetric molecular bonding in adsorption dynamics.