Valence Electron Count Comparison

Steps:

• Determine valence electrons for HCN: H (group 1, 1 electron) + C (group 14, 4 electrons) + N (group 15, 5 electrons) = 10 total.
• Calculate for HNO: H (1) + N (5) + O (group 16, 6 electrons) = 12 total.
• Calculate for NH3: N (5) + 3×H (1) = 8 total.
• Calculate for NO: N (5) + O (6) = 11 total; for PN: P (group 15, 5 electrons) + N (5) = 10 total.

Why D is correct:

• PN totals 10 valence electrons, matching HCN, as both atoms are in group 15 with 5 valence electrons each per periodic table group definitions.

Why the others are wrong:

• A (HNO) has 12 valence electrons due to oxygen's 6.
• B (NH3) has 8 valence electrons from three hydrogens.
• C (NO) has 11 valence electrons from nitrogen and oxygen.

Final answer: D