The first ionization energy of $\mathrm{Na}, \mathrm{Mg}, \mathrm{Al},$ and $\mathrm{Si}$are in the order of

The first ionization potential of $\mathrm{Na}, \mathrm{Mg}, \mathrm{Al},$and $\mathrm{Si}$are in the order,
$\mathrm{Na}<\mathrm{Mg}>\mathrm{Al}<\mathrm{Si}$
On moving from left to right in a period, the ionization energy increases.

$\mathrm{Al}$has lower ionization energy than $\mathrm{Mg}$.
$\mathrm{Al}$has valence shell electronic configuration $3{\mathrm{s}}^{2}3{\mathrm{p}}^1$.
$\mathrm{Mg}$ has valence shell electronic configuration $3{\mathrm{s}}^2$.
Less energy is required to remove a p electron than filled s electron from the same principal quantum shell.