The correct order of first ionisation enthalpies of the following elements is:

B e > M g > C a > S r > R a > B a

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R a > B a > S r > C a > M g > B e

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B e > M g > C a > S r > B a > R e

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R a > S r > B a > M g > C a > B e

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B e > M g > C a > R a > B a > S r

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Solution

The correct option is C $B e > M g > C a > S r > R a > B a$
Ionisation enthalpy decrease on going down the group.
So, $I E \propto \frac{1}{s i z e}$
However, radium has exceptionally higher ionisation enthalpy in group 2.
The first ionisation enthalpy in $k J m o l^{- 1}$ of alkali metal and alkaline earth metals are as following:
$B e = 399; M g = 737$
$C a = 690; S r = 549$
$R a = 509; B e = 503$
Hence, order of ionisation enthalpy is
$B e > M g > C a > S r > R a > B a$

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