A theoretical study of the adiabatic interaction of a metastable 2s hydrogen (or 2^1,3 S helium) atom with an idealized metal surface has been carried out where account has been taken of the 2s-2p state mixing due to the atom metal interaction. The variationally determined wavefunction and resonant-ionization rate are calculated as a function of atom-surface separation. It is found for the 2s(2p) state that the 2s-2p mixing causes a substantial lowering (raising) of the electronic energy and an increase (decrease) in the ionization rate. The competitive process of Penning ionization of adsorbed atoms is discussed using a simple model.