The second ionization energy of alkaline earth metals is less than that of corresponding alkali metals because
First ionization energy values of alkaline earth metals are higher than that of alkali metals, but second ionization energy values of alkaline earth metals are much smaller than those of alkali metals. This occurs because in alkali metals the second electron is to be removed from a cation, which has already acquired a noble gas configuration. In the case of alkaline earth metals the second electron is to be removed from a monovalent cation, which still has one electron in the outermost shell. Thus, the second electron can be removed more easily in the case of group 2 elements than in group 1 elements.