Answer:
Cl, S, Si, Mg, Na
Explanation:
The ionization energy is the minimum amount of energy required to remove the most loosely bound electron of an isolated neutral gaseous atom. In the periodic table, it increases from left to right and from bottom to top.
All the elements are in the same period, so the ionization energy is higher in higher groups.
The order from highest to lowest ionization energy is:
Cl, S, Si, Mg, Na