The first ionization enthalpy of Na, Mg and Si , respectively, are:...

In the third period, $Al$ comes after $Mg$ and before $Si$. Since in $Mg$ ($1{s}^{2}2{s}^{2}2{p}^{6}3{s}^{2}$ ), the last electron is to be lost from a fully filled $3s$-orbital while in $Al$ (${ls}^{2}2{s}^{2}2{p}^{6}3{s}^{2}3{p}^{1}$) it is to be lost from a $3p$-orbital, therefore, ${∆}_{i}{H}_{1}$ of $Al$ is lower than that of $Mg$ ($737kJ{mol}^{- 1}$). Further, nuclear charge increases in going from $Na$ to $Al$, therefore ${∆}_{i}{H}_{1}$ of $Al$ is higher than that of $Na$($496kJ{mol}^{- 1}$) but lower than that of $Mg$ ($737kJ{mol}^{- 1}$). Therefore, the appropriate value of ionisation potential of $Al$ is $577kJ{mol}^{- 1}$.