The image potential between an ion and a metallic surface is calculated via a second-order perturbation approach for various metallic electron densities varying from rs = 2 to 6. The results are compared with the classical image potential –e^2/4z. A critical distance zc, for each rs, is determined such that for z < zc the classical result is no longer valid. The importance of the image potential and zc in the theory of charge transfer in atom (ion)-surface collisions is discussed.