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Q.
The first ionization enthalpies of $Na$, $Mg$, $Al$ and $Si$ are in the order
Classification of Elements and Periodicity in Properties
Solution:
As we move across the period, atomic size decreases hence, ionization enthalpy increases. But for $Al\, (3s^23p^1)$, electron has to be removed from outer $3p$ orbital whereas in $Mg(3s^2)$, electron has to be removed from stable fully filled $3s$ orbital. Thus, I.E. for $Mg > Al$. So, the correct order of first ionization enthalpies is : $Na < Mg > Al < Si$.