The question is "Why is the third ionisation energy of magnesium higher than that of fluorine?"
Although I understand that differences in ionisation energy have several factors governing it, e.g. the shielding effect, distance from the nucleus, whether the electron is in its own orbital or paired with another electron etc.
The answer to this question is that "Even though the valence electrons of both elements are removed from the same shell, Mg has greater nuclear charge."
I am confused what the answer means about nuclear charge. Doesn't the third ionisation energy of Mg and F have the same nuclear charge as they are both 3+?