Ionization Energy Trends: Arranging Ar, Na, Cl, and Al

Na < Al < Cl < Ar

This is because ionization energy generally increases as you move from left to right across a period (due to increasing nuclear charge) and decreases as you move down a group (due to increasing atomic size). Therefore, Na has the lowest ionization energy among the given elements because it is in the same group as highly reactive metals such as Li and K, which have low ionization energies. Al has a slightly higher ionization energy because it is to the right of Na in the same period. Cl has an even higher ionization energy because it is further to the right in the same period, and Ar has the highest ionization energy because it is a noble gas with a full valence shell.