In this paper, we present a detailed mechanism for the complete decomposition of NH3 to NHx(a) (x=0–2). Our calculations show that the initial decomposition of NH3 to NH2(a) and H(a) is facile, with a transition‐state energy 7.4 kcal mol−1 below the vacuum level. Further decomposition to N(a) or recombination–desorption to NH3(g) is hindered by a large barrier of ∼46 kcal mol−1. There are two plausible NH2 decomposition pathways: 1) NH2(a) insertion into the surface SiSi dimer bond, and 2) NH2(a) insertion into the SiSi backbond. We find that pathway (1) leads to the formation of a surface SiN unit, similar to a terminal SiNt pair in silicon nitride, Si3N4, while pathway (2) leads to the formation of a near‐planar, subsurface Si3N unit, in analogy to a central nitrogen atom (Nc) bounded to three silicon atoms in the Si3N4 environment. Based on these results, a plausible microscopic mechanism for the nitridation of the Si(100)–(2×1) surface by NH3 is proposed.