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H2 adsorption and desorption are fundamental process in the growth of all materials. In this talk we describe how the surface radicals or dangling bonds (DBs) that are generated by H2 desorption from Si(100) behave under high temperature growth conditions. Initially these DBs are found on the same surface dimer unit but eventually DB dissociation, diffusion and recombination occur. These processes are monitored in real time by STM. An analysis of this behavior shows that there is a weak binding interaction between DBs on the same dimer but that this is readily overcome by entropic effects at higher temperatures. We will also address the controversial topic of H2 on the Si(100) surface. Detailed balance arguments indicate that the barrier to adsorption should be vanishingly small. Despite this, the sticking coefficient is less that 10-12 at room temperature. Using STM we show that this discrepancy is due to the neglect of the motion of surface Si atoms during the adsorption process. By specially preparing a Si(100) surface with "frozen dimers" we demonstrate that the sticking coefficients can be as high as 10-4 at room temperature. The implication of these observations for the structure of the transition state and the potential energy surface along the reaction co-ordinate, are discussed.