# The 1st ionization potential Na,Mg,Al and Si are :

$(a)\;Na < Mg > Al < Si \\(b)\; Na > Mg > Al > Si\\ (c)\;Na < Mg < Al > Si \\ (d)\;Na > Mg > Al < Si$

The correct order is $Na < Mg > Al < Si$
Mg has greater ionization energy than aluminium because Mg has stable electronic configuration with completely filled 3s orbital whereas in Al the 3p orbitals are incompletely filled. Moreover the penetration power of 3s orbitals is more than 3p therefore, it is difficult to remove electrons from 3s orbital.
Hence a is the correct answer.
edited Nov 11, 2017