The energy required to remove an electron completely from the outermost orbit of an isolated gaseous atom is called the ionization potential of the element. Across a period, first ionization potential increases except few atoms of elements. Here it is to be remembered that removal of an electron from a p-orbital is easier than from a completely filled 5-orbital. So, the first ionization potential of Al is less than Mg. So, the correct sequence is: $ Na < Al < Mg < Si $