Final answer:
Magnesium has a greater first ionization energy than Calcium largely due to Calcium's stronger electron shielding effect, which is influenced by an additional electron shell that decreases the effective nuclear charge experienced by the outer electrons.
Step-by-step explanation:
The question is why the first ionization energy of Magnesium (Mg) is greater than that of Calcium (Ca). The amount of energy required to remove an electron from an atom is referred to as ionization energy. Ionization energy generally increases across a period due to the increasing nuclear charge; however, other factors such as atomic size and electron shielding can influence these trends.
For Mg and Ca, the correct explanation for Mg having a higher first ionization energy than Ca is due to electron shielding. Calcium has more protons than Magnesium, which typically would imply a higher ionization energy for Ca. However, Calcium also has an additional electron shell compared to Magnesium. This extra shell increases the distance between the outer electrons and the nucleus and also adds more electron-electron repulsion, which lessens the hold of the nucleus on the outer electrons. This effect is known as electron shielding. Therefore, the answer is D) Calcium has a stronger electron shielding effect, which results in a lower first ionization energy compared to Magnesium.