The stability of Cu(001)-(2x1)-Li and Al(001)-c(2x2)-Na systems, in which the adatoms (Li or Na) occupy substitutional sites are investigated based on first-principles electronic structure calculations. The total energies of the systems are calculated and compared, by considering some related surface structures and optimizing the structural parameters involved. The stability is discussed in terms of vacancy-formation and binding energies. Change in work function by adsorption is also discussed.