Unpaired vs. paired electrons in n=2 shell via Hund's rule

Steps:

• Determine ground-state configurations: C (1s²2s²2p²), N (1s²2s²2p³), O (1s²2s²2p⁴), F (1s²2s²2p⁵).
• For n=2 (2s² paired for all; 2p varies), apply Hund's rule: singly occupy orbitals before pairing.
• Count in n=2: C (2 unpaired, 2 paired), N (3 unpaired, 2 paired), O (2 unpaired, 4 paired), F (1 unpaired, 6 paired).
• Select atom where unpaired > paired: only nitrogen.

Why B is correct:

• Nitrogen's 2p³ has three unpaired electrons (one per orbital) and 2s² has two paired, totaling more unpaired than paired in n=2.

Why the others are wrong:

• A. Carbon's 2p² has two unpaired and 2s² two paired; numbers equal.
• C. Oxygen's 2p⁴ has two unpaired (two singles) and two paired (one pair), plus 2s² (two paired); more paired.
• D. Fluorine's 2p⁵ has one unpaired (one single) and four paired (two pairs), plus 2s² (two paired); more paired.

Final answer: B