[en] The dependence on the nuclear charge of exponents minimizing the total energy of minimum basis ground state wavefunctions for first- and second-row atoms and some Ne-like and Ar-like ions is examined. Excellent linear correlations are found with correlation coefficients better than 0.999. Interpretation within a model of screened nuclear charges suggests, in agreement with the classical Slater rules, that s and p electrons with the same n are grouped together and that screening due to outer-shell electrons is negligible. In disagreement with these rules, it is found that a multiplying factor different from unity is necessary for 2s and 3s exponents to correct for deficiencies in the functional form of the ns atomic orbitals