Electron Affinities of the Alkaline Earth Metals and the Sign Convention for Electron Affinity

Most general chemistry textbooks published in the last six years contain tables or figures showing the alkaline earth metals with large endothermic electron affinities. These data come from outdated extrapolations and are seriously in error. It has been known since 1987, both theoretically and experimentally, that the ion Ca^- is stable. It is now certain that Sr^-, Ba^-, and Ra^- are also stable, and accurate values for the electron affinities of Ca^-, Sr^-, and Ba^- have been determined. Recommended values for these electron affinities, in the units commonly employed in introductory texts and with the sign convention used here, are 2.37, 5.03, and 13.95 kJ/mol for Ca, Sr, and Ba, respectively. The endothermic electron affinities often quoted for Be and Mg are also too large and should be reported simply as ">0". An argument for a return to the original sign convention for the electron affinity is presented in this paper.