Periodic trends explain ionization energy differences

Steps:

• Ionization energy increases across periods (higher nuclear charge) and decreases down groups (increased shielding).
• Student 2 is correct if X has smaller radius than Y, as closer electrons are harder to remove.
• Student 1 likely means X has more protons (higher effective nuclear charge), which raises IE.
• Test options: only one student's reason must hold true for the pair.

Why D is correct:

• In group 16, oxygen (X) has higher first IE than sulfur (Y) per the trend of decreasing IE down a group; O's smaller radius (due to less shielding) supports Student 2, while O has fewer protons (8 vs. 16), making Student 1 wrong.

Why the others are wrong:

• A: Carbon has more protons (6 vs. 5) and smaller radius than boron, so both students correct.
• B: Magnesium has higher IE than aluminum but fewer protons (12 vs. 13) and larger radius, so both wrong.
• C: Aluminum has lower IE than magnesium, violating the question premise.

Final answer: D