Abstract Charge and spin states of metastable He atoms and positive He ions approaching metal surfaces are investigated with the aid of a simple time-dependent model Hamiltonian. With a simple decoupling approximation, a closed set of Heisenberg equations of motion is derived for the time-dependent quantities describing the charge and spin states of the He atoms. The numerical calculations in the case of simple time-dependence for the He atom-metal interaction, show that the main fraction of incident metastable He atoms changes into positive He ions near the metal surfaces if the work function of the surfaces is high and the Fermi level is located relatively far below the energy level for the 2s orbital of the He atoms. The singlet-triplet conversion of incident metastable He atoms as well as the conversion of the incident ions into metastable He atoms occurs efficiently near the surfaces if the work function gets lower and the Fermi level comes nearer to the energy level for the 2s orbital. However, if the work function gets even low and the Fermi level is located relatively far above the 2s level, the characteristic time for the singlet-triplet conversion becomes considerably long.