Upon nitrogen adsorption at low temperature onto the surface of magnesium oxide and calcium oxide containing F-S(+) centers (single electron trapped in a suitable surface vacancy), electron transfer occurs from the solid to the adsorbed molecule. About 90% of the total electron density is localized on the adsorbed molecule. The Ii-electron N-2(-) radical anion so formed has been detected by electron paramagnetic resonance for both N-14(2) and N-15(2). The electron transfer is reversible and, when the pressure is lowered or the temperature increased, the Nz molecules desorb, regenerating the original F-S(+) center. The diatomic radical ion lies parallel to the surface, and the electron density is mainly confined in the pi (y)* orbital. Theoretical calculations at the DFT level indicate that a small energy barrier separates the unbound F-S(+)/N-2 state from the bound F-S(2+)/N-2(-) state. This explains the facile reversibility of the electron-transfer process. The calculated spin densities are in excellent agreement with those derived from the experiments. The results reported in this paper represent a new method for N-2 activation over the basic alkaline earth oxides, which are also known to activate H-2 by dissociative adsorption.