Final answer:
Chlorine has a higher ionization energy than magnesium because its valence shell is nearly full, making it more difficult to remove an electron compared to magnesium, which has two valence electrons easier to remove. So the best answer is Cl; it takes much more energy to pull an electron away from a valence shell that is nearly full.
Step-by-step explanation:
Chlorine (Cl) has a higher ionization energy than magnesium (Mg). This is because it takes much more energy to remove an electron from a valence shell that is nearly full, as in chlorine, compared to removing an electron from an atom that has a less filled valence shell, like magnesium. Chlorine's valence shell is nearly full, needing only one more electron to complete its octet, which means its nucleus holds onto its electrons more tightly. Conversely, magnesium's ionization energy is lower because it has two valence electrons that are relatively easier to remove as magnesium aims to achieve a noble gas configuration.
Moreover, the ionization energies increase drastically when electrons are removed from a lower energy level than the outermost, which would be the case after magnesium loses its initial two valence electrons. For chlorine, however, the ionization energy remains high because its electrons are all in the third shell, and the additional electron would complete its octet, making the removal of an electron extremely difficult due to strong electrostatic attraction to the nucleus.