Question

Alkaline earth metals always form divalent cations even though their second ionisation enthalpies are almost double than their first ionisation enthalpies

Answer 1

Alkali metals have one electron in its outermost shell while alkaline earth metals have two electrons in their outermost shell. Therefore, alkali metals by losing one electron attain full filled electronic configuration, so they lose electron easily and thus the first ionisation enthalpy is low in comparison to alkaline earth metals which need the removal of two electrons to attain full filled electronic configuration.

After losing one electron, the alkali metals attain noble gas configuration and to remove electron from a fully filled shell requires huge amount of energy, while in case of alkaline earth metals, removing second electron from the shell requires relatively lesser amount of energy as after removing two electrons these metals will attain noble has configuration.Thus the second ionisation enthalpy of alkaline earth metals is smaller than that of the corresponding alkali metals.