Final answer:
Phosphorus (P) would have the highest ionization energy among Na, Mg, Al, and P because it is the furthest to the right and highest in its period, indicating it would be more difficult to remove an electron due to its position in the periodic table.
Step-by-step explanation:
The question focuses on predicting which element among Na, Mg, Al, and P would have the highest ionization energy. Ionization energy typically increases across a period (from left to right) and decreases down a group (from top to bottom) in the periodic table. Therefore, phosphorus (P), being furthest to the right and highest up in its period among the options, would generally have the highest ionization energy.
The first ionization energy of Mg is higher than that of Al because Mg has a complete 3s2 subshell, which is more difficult to remove an electron from compared to Al's 3s23p1 configuration, where the single p electron is easier to remove.