A Levels Chemistry (9701)•9701/11/M/J/18
Explanation
Periodic trends in atomic radius and ionization energy
Steps:
- Locate each element: Boron (group 13, period 2), sodium (group 1, period 3), magnesium (group 2, period 3), calcium (group 2, period 4).
- Atomic radius decreases up the group; second smallest means second lightest in group 2 (Be smallest, Mg next).
- First ionization energy (IE) in period 3 increases overall left to right with exceptions: Na (496 kJ/mol, lowest), Al (578, second), Mg (738, third lowest).
- Magnesium fits both: second smallest radius in group 2, third lowest IE in period 3.
Why C is correct:
- Magnesium is second in group 2 (after Be) for radius and third lowest IE in period 3 per experimental values.
Why the others are wrong:
- A. Boron: smallest radius in group 13, not second; second lowest IE in period 2.
- B. Calcium: third smallest radius in group 2; fourth lowest IE in period 4.
- D. Sodium: second smallest radius in group 1; lowest IE in period 3.
Final answer: C
Topic: The Periodic Table: chemical periodicity
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