Question

Second ionization energy of alkali metals is very high whereas that of alkaline earth metals is low

Answer 1

Alkali metals contain only one valence electron.On loosing this electron, these metals attain noble gas configuration. Therefore, the removal of second electron requires huge amount of energy.Hence, the second ionization energy of alkali metals is very high.
On the other hand, alkaline earth metals contain 2 valence electrons.On loosing the second electron, these metals attain noble gas configuration.Hence, the second ionization energy of alkaline earth metals is low.