Which of the following ABN-type molecules has a trigonal pyramidal molecular geometry?

• A. BF3
• B. NH3
• C. CH4
• D. CO2

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the ABN-type molecule notation, where 'A' is the central atom, 'B' represents bonded atoms, and 'N' represents lone pairs on the central atom. The molecular geometry depends on the number of bonded atoms and lone pairs around the central atom.
2. Step 2: Recall that trigonal pyramidal geometry occurs when the central atom has three bonded atoms and one lone pair, resulting in a shape similar to a pyramid with a triangular base.
3. Step 3: Analyze each molecule's Lewis structure to determine the number of bonded atoms and lone pairs on the central atom: BF3 has 3 bonded atoms and 0 lone pairs; NH3 has 3 bonded atoms and 1 lone pair; CH4 has 4 bonded atoms and 0 lone pairs; CO2 has 2 bonded atoms and 0 lone pairs.
4. Step 4: Use VSEPR theory to predict the molecular geometry based on the electron pair arrangement: BF3 is trigonal planar, NH3 is trigonal pyramidal, CH4 is tetrahedral, and CO2 is linear.
5. Step 5: Conclude that the molecule with trigonal pyramidal geometry is the one with three bonded atoms and one lone pair on the central atom, which is NH3.