The adsorption of alkali-metal atoms (M = Li, Na, and K) on the surface of MgO has been studied by means of embedded-cluster DFT calculations. Alkali-metal atoms bind preferentially to the oxide anions with energies of the order of 1 eV. On these sites the ns valence electron remains localized on the alkali atom, but the substantial polarization (ns-np mixing) leads to major changes in the isotropic hyperfine coupling constants. In the presence of specific defect sites, like a bare oxygen vacancy, F-S(2+) center, a net charge transfer occurs, with formation of F-S(+) color centers. At higher coverages, once the F-S(2+) centers have been saturated, a different process takes place. At specific neutral morphological defects, like a cationic reverse corner, the alkali atom valence electron is transferred to the surface with formation of M+(e(-))(trapped) pairs. The computed properties of the unprecedented W(e)trapped pairs (hyperfine constants and optical transitions) are consistent with the experimental measures and show that the trapped electron and the adsorbed alkali-metal cation are separated by short distances.