[en] The dependence of the electronic stopping cross-section on the atomic number of the target material has been studied on the basis of binary stopping theory over a wide range of beam energies. A distinct dependence on ion type and energy is found for the predicted Z₂-structure which becomes significantly less pronounced for screened heavy ions than for protons and antiprotons. This behavior is caused primarily by the interplay between projectile screening and closing of inner target shells as a function of the beam velocity and introduces a pronounced Z₂-structure in the effective-charge ratio. Related effects are found for stopping in three different phases of carbon and in lithium fluoride