Atomic nitrogen on the Si(100)-(2 x 1) surface is investigated using B3LYP density functional theory to study the incorporation of nitrogen atom into the silicon surface during growth of nitride films on silicon. Several possible structures for nitrogen on the Si(100)-(2 x 1) surface are investigated, including N bridge-bonded into the Si-Si dimer, N bridge-bonded into the Si-Si back-bond, and N inserted by forming three bonds with three Si atoms. Furthermore, the energetics and reaction mechanisms leading to these structures are also calculated. We find that the structure with nitro-en atom bridge-bonded into the Si-Si dimer to be the most thermodynamically stable, with an adsorption energy of 105 kcal/mol and an insertion barrier of 29 kcal/mol. Insertion into the Si-Si back-bond, however has the lowest activation barrier of 11 kcal/mol, although its adsorption energy is 13 kcal/mol lower than insertion into the Si-Si dimer. The third configuration investigated with N forming three bonds with Si atoms has a relatively high activation barrier of 35 kcal/mol and an adsorption energy of 95 kcal/mol. In addition, the formation of this structure "consumes" three Si atoms. Hence, at high nitrogen pressure, this structure is expected to be less dominant than those with N inserted into the Si-Si bonds, which only consume two Si atoms.