Assertion : The ionisation enthalpies of alkaline earth metals are fairly low.
Reason : In general, the low ionisation enthalpies of alkaline earth metals are due to large size of their atoms.
(A). Alkali metals have only one valence electron, while alkaline earth metals have two valence electrons. After the only outermost electron is removed from the atom of an alkali metal, the ion formed has a stable configuration of the nearest noble gas. Removal of another electron from the unipositive ion of alkali metal, therefore, would require very large amount of energy.