The s-Block Elements

i) The alkaline earth metals owing to their large size of atoms have fairly low values of ionisation enthalpies. Within the group, the ionisation enthalpy decreases as the atomic number increases. It is because of increase in atomic size due to the addition of new shells and increase in the magnitude of screening effect of the electrons in inner shells.

ii) The first ionisation enthalpies of alkaline earth metals are higher than alkali metals because of smaller atomic size and higher than alkali metals because of smaller atomic size and higher effective nuclear charge. Second ionisation enthalpy  is smaller than that of alkali metals because alkali metals acquire noble gas configuration after losing one electron. 

This is because the atoms of alkaline earth metals have a smaller size as compared to the alkali metals. Therefore, the electrons in the outermost shells of the alkaline earth metals are more tightly held. As a result, the ionisation enthalpies are higher than those of alkali metals.

Alkali metals have only one valence electron, while alkaline earth metals have two valence electrons. After the removal of one electron from alkali metals, they form M⁺which is a stable noble gas configuration. Removal of another electron from the unipositive ion of alkali metal, therefore, would require a very large amount of enthalpy.
On the other hand, after the removal of one electron from alkaline earth metals, they do not acquire stable noble gas configuration and still have a tendency to lose another electron. As a result, the second ionisation enthalpies of alkali metals are much higher than those of the alkaline earth metal. 

Alkaline earth metals are fairly electropositive as their atoms have a tendency to lose their two outermost electrons forming dipositive ions. 

On moving down the group, the atomic radii increase and ionisation enthalpies decrease. Consequently, the electropositive or metallic character increases. Thus, Mg is more electropositive than Be and so on.