Why the electron gain enthalpy values of alkaline earth metals are lower (i.e. less negative) or positive?

Explanation

The general valence shell electron configuration of alkaline earth metals is `ns^(2)` (stable configuration). The extra electron must enter np subshell, which is effectively shielding by the two ns electrons and the inner electrons. Consequently, the alkaline earth metals have little or no tendency to pick up an extra electron.