Recently Davidson [ital et] [ital al]. [Phys. Rev. A 44, 7071 (1991)] have estimated nonrelativistic correlation energies and relativistic corrections to ionization potentials for atomic ions with up to 10 electrons. In this work, this approach is extended to atomic ions with 11 to 18 electrons. The correlation energies for 3- to 10-electron atomic ions are also recomputed using more recent experimental and theoretical data. Unlike other work the method focuses on the correlation contribution to the individual ionization energies which are obtained by comparing experimental data with relativistic complete-valence-space energies. [ital Ab] [ital initio] estimates of correlation contributions to the ionization energies with extensive configuration-interaction calculations of 3- to 10-electron atomic ions with nuclear charge from 4 through 10 and 18, 36, 50, 72, 100, and 144 have been obtained. The correlation energies obtained from some density-functional models are also compared to these correlation energy data.