We have investigated the interactions of several adsorbates, Al, C, Ge and Mg, with the Si(100) 2*1 surface using a semiempirical molecular orbital method. Our results show that elements with a smaller electronegativity than that of the substrate (Si), such as Al and Mg, lose charge to the substrate, whereas C and Ge, which both have a higher electronegativity, gain charge. These findings are compatible with the experimental studies on the phenomena of electromigration on the Si(100) 2*1 surface where different adsorbates migrate in different directions with respect to the electron flow on the surface. A more detailed study is made on the adsorption and diffusion properties of Al on the Si(100) surface. Al is found to be highly mobile with a preferred diffusion path perpendicular to the dimer rows. For diatomic Al, our calculations favour the parallel ad-dimer model rather than the orthogonal model, in agreement with the findings of Northrup et al. (1991) and Itoh et al. (1993). In addition, for Al adsorption in the vicinity of a missing dimer, the atomic configuration of the terminating Al atom at the end of the Al dimer line is compatible with that observed under the STM.