Chemisorption, by definition, involves the formation of new chemical bonds between the adsorbed species and the surface atoms of the substrate - basically the same type of bonds that are present in any molecular complex. In considering what type of species may be formed on a metal surface, therefore, it is important not to abandon chemical common sense and, if in doubt, to look for inspiration at the structures of known metal-organic complexes. The adsorbed species in this instance therefore are hydrogen atoms.
Halogens also chemisorb in a dissociative fashion to give adsorbed halogen atoms.
Clearly the kinetic barrier to dissociation must also be low or non-existent for the dissociative adsorption to occur readily. The net result of these two effects when taken to their limit is that the halogen molecule dissociates and the halogen atoms interact with the metal with a strong ionic contribution to the bonding. As a result of the electron transfer from the metal substrate to the halogen atoms, each adsorbed atom is associated with a significant surface dipole. Another feature of the halogen adsorption chemistry of some metals is the transition from an adsorbed surface layer to surface compound formation at high gas exposures. In those cases where both types of adsorption are observed it is the dissociative process that corresponds to the higher adsorption enthalpy.