Choose the correct order of Ist ionisation energy.

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$\begin{aligned} O > N \end{aligned}$

$\begin{aligned} Ne < F \end{aligned}$

$Na > Al$

$Mg > Al$

answer is D.

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Detailed Solution

Nuclear charge rises, atomic size falls, and ionisation enthalpy rises as we travel through the period. For $Al (3 s^{2} 3 p^{1})$ it is necessary to remove an electron from a partially filled 3p orbital. On the other hand, $Mg (3 s^{2})$ in the electron in the stable, fully occupied 3s orbital needs to be removed. More energy is needed to remove an electron from a stable, fully filled orbital than from a partially filled orbital. As a result, the ionisation enthalpy for magnesium is higher than that of aluminium. Hence, the correct order of Ist ionisation energy is $Mg > Al$ .