Final answer:
Magnesium has a greater second ionization energy than strontium because the electron being removed from magnesium is from a more stable, lower energy level closer to the nucleus, compared to the larger, more shielded electron configurations found in strontium.
Step-by-step explanation:
Magnesium has a greater second ionization energy than strontium because the electron being removed comes from a more stable, lower energy level and a closer shell to the nucleus. Magnesium's second ionization involves removing an electron from the 3s subshell, which is closer to the nucleus and thus more strongly bound than the electrons in strontium's corresponding subshell. As atoms become larger down a group, the outer electrons are less tightly held due to increased shielding and distance from the nucleus. Hence, strontium, being below magnesium in the periodic table, has a larger atomic radius, which results in a lower second ionization energy compared to magnesium.
The trend in ionization energy typically increases as electrons are removed from an atom because the same number of protons in the nucleus attracts fewer electrons. Additionally, when a successive ionization energy requires removing an electron from a completely new shell, as with magnesium's third IE, the amount of energy needed jumps significantly because it involves removing an electron from a lower, more tightly bound energy level.