The hint given on my Sapling Learning homework when I got it wrong by entering the common trend answer (Ar, Cl, S, P, Si, Al, Mg, Na) was something along the lines of "ns^2 is full and somehow has more ionization energy. ns^2p^3 also has more IE." Trend base is halogen (highest) and alkaline (lowest) with these exceptions. Mg is the full ns^2 and P is the ns^2p^3. Not sure how to figure these out without an IE chart.
Correct IE order (highest to lowest): Ar, Cl, P, S, Si, Mg, Al, Na
This is the correct answer for Rank these elements according to first ionization energy. Highest to lowest. Ar, Na, Al, P, Cl, Si, S, Mg