A simple model of the thermal dihalogenation of a silicon surface by an adsorbate, Br-R-Br, is described for the first time. The model consists of a pair of silyl radicals (SiH3), treated by density functional theory, placed a distance d* apart. Two adsorbates were considered, one with R = benzene and one with R = biphenyl. The Br … Br internuclear separation (r(Br, Br)) in 1,4-dibromobenzene is 6.6 Å, whereas that in 4,4′-dibromobiphenyl is 10.9 Å. In the former case the model yields a (most-favourable) ‘critical’ separation d*≈ 10.2 Å, corresponding to the lowest-energy barrier, and in the latter case d*≈ 13.8 Å. The dangling-bond separations, d*, were restricted to values present at an Si(111)7×7 surface. The critical values, d*, exceeded the corresponding value of r(Br, Br) in the intact adsorbate in each case by ca. 3.5 Å, corresponding to ca. 93% of the van der Waals molecular size. This is approximately the observed increase in separation of a pair of halogen atoms in reactions at a Si surface, as compared with the separation in the parent molecule. If the dangling bonds at the surface are too close to one another (d < d*), the adsorbate molecules' charge cloud extends too far for a favourable interaction, whereas if the dangling bonds are too far apart (d > d*) the adsorbate's charge cloud cannot reach them. When the second reagent is a peg-board, the factors governing chemical reactivity are more exposed to view.