Density functional calculations which explicitly use the density gradient type nonlocal corrections to exchange and correlation are reported for the adsorption of atomic nitrogen and oxygen on the four-fold sites of (100) copper and silver surfaces simulated by Cu, and Ag, clusters. Binding energies, equilibrium distances and harmonic vibrational frequencies are in good agreement with the available experimental and high level theoretical data. An ionic contribution is found in the bonds between N and 0 and Cu and Ag clusters. Metal d orbitals are involved in the covalent part of the bond.