The activation of ammonia by cerium atom has been investigated in solid argon using infrared spectroscopy and density functional theoretical calculations. The results reveal that the spontaneous formation of CeNH3 complex on annealing is the initial step in the reactions of cerium atoms with ammonia. The CeNH3 complexes rearrange to generate the inserted HCeNH2 molecules on irradiation. A "triplet-singlet" spin conversion occurs along the reaction path in which HCeNH2 (3A″) isomerizes into H2CeNH (1A'). The H2CeNH molecules finally decompose to yield HCeN + H2 upon photolysis. The periodic trend and differences for the M + NH3 (M = Ti, Zr, Hf, Ce, Th) systems are discussed on the basis of the present and previous works. DFT calculations predict that the most stable ground state for HHfNH2 and HThNH2 is singlet due to the stronger relativistic effects in Hf and Th atoms, while that for HTiNH2, HZrNH2, and HCeNH2 is triplet. Besides, the H2-elimination process is different for Ce and M (Ti, Zr, Hf, Th) cases.