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  4. The correct order of first ionisation enthalpies of the following elements is

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Q. The correct order of first ionisation enthalpies of the following elements is

KEAMKEAM 2015The s-Block Elements

Solution:

Ionisation enthalpy decrease on going down the group.

So, $IE \propto \frac{1}{\text { size }}$

However, radium has exceptionally higher ionisation enthalpy in group $2.$

The first ionisation enthalpy in kJ $mol ^{-1}$ of alkali metal and alkaline earth metals are as following:

$Be =899 ;\, Mg =737$

$Ca =590 ;\, Sr =549$

$Ra =509 ;\, Ba =503$

Hence, order of ionisation enthalpy is

$Be > Mg > Ca > Sr > Ra > Ba$