Ab initio G2M(MP2)//MP2/6-31G** calculations have been performed to study the molecular and radical chain reaction mechanisms of nitrogen hydrogenation through sequential additions of three H-2 molecules to N-2 producing NH3 + NH3. All reaction steps of the molecular mechanism are shown to be slow owing to high barriers for the molecular hydrogen additions. The three-center 1,1-HZ additions are significantly more preferable as compared to the four-center 1,2-additions. The most favorable reaction pathway involves the steps N-2 + H-2 --> TS1a --> NNH2, NNH2 + H-2 --> TS3a --> H2NNH2, H2NNH2 --> TS4 --> HNNH3, and HNNH3 + H-2 --> TS5 --> NH3 + NH3, with the barriers calculated as 125.2, 30.7, 60.5, and 24.6 kcal/mol, respectively. The addition of the first molecular hydrogen is thus the rate-determining stage of nitrogen hydrogenation. The formation of hydrazine can be facilitated by a spontaneous reaction of two cis-HNNH molecules by the dihydrogen transfer mechanism. The radical chain mechanism includes the N-2 + H --> N2H, N2H + H-2 --> HNNH + H, HNNH + H --> N2H3, N2H3 + H-2 --> H2NNH2 + H, H2NNH2 + H --> NH2 + NH3, and NH2 + H-2 --> NH3 + H sequential reactions with the barriers of 17.1, 41.6, 6.4, 29.1, 10.7, and 10.6 kcal/mol, respectively. Nitrogen hydrogenation can be catalyzed by H atoms with the barrier for the slowest reaction step decreasing from 125 to 42 kcal/mol. The reaction of two NH((3)Sigma(-)) radicals is predicted to be fast and to form N-2 + H-2 with high exothermicity. The reaction of two NH2 radicals can produce NNH2 + H-2 with exothermicity of 19.8 kcal/mol and a barrier of 10.9 kcal/mol relative to the reactants, or NH3 + NH((3)Sigma(-)), through a barrierless, 14.3 kcal/mol exothermic, but spin-forbidden channel. We also report rate constants and equilibrium constants for all considered reactions calculated using the transition state theory and ab initio energies and molecular parameters, which can be employed for kinetic modeling of chemical processes involving nitrogen- and hydrogen-containing substances.