Which of the following best describes the resonance structures of $HCN$ (hydrogen cyanide)?

• A. $H-C=N$ and $H=C-N$ are both significant resonance structures for $HCN$.
• B. $HCN$ has multiple resonance structures with delocalized electrons over all three atoms.
• C. $H-C\equiv N$ and $H=C=N$ are equally important resonance structures.
• D. $H-C\equiv N$ is the only significant structure; $HCN$ does not have significant resonance structures.

Answer: (D)

Verified step by step guidance

1. Step 1: Draw the Lewis structure of hydrogen cyanide (HCN). Hydrogen (H) is bonded to carbon (C), which is bonded to nitrogen (N). Carbon has 4 valence electrons, nitrogen has 5, and hydrogen has 1, so arrange electrons to satisfy the octet rule for C and N and duet for H.
2. Step 2: Consider possible resonance structures by moving electrons (pi bonds and lone pairs) without breaking the connectivity of atoms. For HCN, check if shifting a double bond or lone pair can create alternative valid Lewis structures.
3. Step 3: Evaluate the resonance structures: one common structure is H-C≡N (a triple bond between C and N), and another possible but less significant is H-C=N with a lone pair on N. However, the triple bond structure is the major contributor due to formal charges and octet satisfaction.
4. Step 4: Analyze the significance of each resonance structure by calculating formal charges and checking octet completion. The structure with minimal formal charges and full octets is the most stable and significant.
5. Step 5: Conclude that H-C≡N (with a triple bond between C and N) is the dominant resonance structure, and other resonance forms like H-C=N with a single bond are not significant. Therefore, HCN does not have multiple significant resonance structures with delocalized electrons over all three atoms.