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 11-electron N₂^- radical anion so formed has been detected by electron paramagnetic resonance for both ¹⁴N₂ and ¹⁵N₂. The electron transfer is reversible and, when the pressure is lowered or the temperature increased, the N₂ 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 π_y* orbital. Theoretical calculations at the DFT level indicate that a small energy barrier separates the unbound F_S⁺/N₂ state from the bound F_S²⁺/N₂^- 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 N₂ activation over the basic alkaline earth oxides, which are also known to activate H₂ by dissociative adsorption.