Reactions of (14) atomic lanthanide cations (excluding Pm+) with ammonia have been surveyed in the gas phase by using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer to measure rate coefficients and product distributions in He at 0.35 +/- 0.01 Torr and 295 +/- 2 K. Primary reaction channels were observed corresponding to H-2 elimination with formation of the protonated lanthanum nitride and NH3 addition. H-2 elimination was seen only in the reactions with La+, Ce+, Gd+, and Tb+ and occurs with these ions exclusively. NH3 addition was seen with Pr+, Nd+, Sm+, Eu+, Dy+, Ho+, Er+, Tm+, Yb+, and Lu+. Higher-order sequential addition of up to five NH3 molecules was observed with the Ln(+)(NH3) and LnNH(+) ions. The reaction efficiency of the primary reactions is seen to decrease as the energy required to promote an electron to make two non-f electrons available for bonding increases. The periodic trend in reaction efficiency along the lanthanide series matches quite closely the periodic trend in the electron-promotion energy required to achieve a d(1)s(1) or d(2) excited electronic configuration in the lanthanide cation. With La+, Ce+, Gd+, and Tb+, the electrostatic attraction between the atomic lanthanide cation and ammonia is sufficiently strong to provide enough energy to achieve electron promotion and to overcome any barriers to subsequent N-H bond insertion and H-2 loss, but this is not the case with the other lanthanide cations with which collisional stabilization of the intermediate adduct ion, with or without insertion of Ln(+), predominates.