Which of the following best describes the molecular geometry of HCN (hydrogen cyanide)?

• A. Trigonal planar
• B. Tetrahedral
• C. Linear
• D. Bent

Answer: (C)

Verified step by step guidance

1. Step 1: Draw the Lewis structure of HCN. Hydrogen (H) is bonded to carbon (C), which is triple bonded to nitrogen (N). This gives a linear arrangement of atoms: H-C≡N.
2. Step 2: Count the regions of electron density around the central atom, which is carbon in this case. Carbon has two regions of electron density: one single bond to hydrogen and one triple bond to nitrogen.
3. Step 3: Use the VSEPR (Valence Shell Electron Pair Repulsion) theory to determine the molecular geometry. Two regions of electron density around the central atom will arrange themselves to be as far apart as possible, which is 180° apart.
4. Step 4: Recognize that with two regions of electron density and no lone pairs on the central atom, the molecular geometry is linear.
5. Step 5: Conclude that the molecular geometry of HCN is linear, not trigonal planar, tetrahedral, or bent.