The dependence of energy of an ion accelerated from a thin laser target on its charge is investigated numerically and analytically. It is demonstrated that this dependence is determined by the values of ionization potentials of the target atoms and has a sharper character than those obtained in previous models of ion acceleration. The physical origin of sharper dependence is the relation of ion charge to its initial position in the ionized target. As a result, more highly charged ions turn out to be in the region of stronger accelerating field and thus attain higher energy. For a laser target made of a gold foil, the results obtained with an analytical model are compared to those obtained by means of a two-dimensional simulation, which demonstrated agreement between theoretical and calculated dependences of ion energy on its charge.