Which of the following best describes the difference between trigonal planar and trigonal pyramidal molecular geometries?

• A. Trigonal planar has four atoms bonded to a central atom, while trigonal pyramidal has three atoms bonded to a central atom with no lone pairs.
• B. Trigonal planar and trigonal pyramidal both have one lone pair on the central atom.
• C. Trigonal planar has three atoms bonded to a central atom with no lone pairs, while trigonal pyramidal has three atoms bonded to a central atom with one lone pair.
• D. Trigonal pyramidal has a bond angle of 120°, while trigonal planar has a bond angle of 107°.

Answer: (C)

Verified step by step guidance

1. Understand that molecular geometry depends on the number of atoms bonded to the central atom and the number of lone pairs on the central atom.
2. Recall that trigonal planar geometry involves three atoms bonded to the central atom with no lone pairs, resulting in a flat, triangular shape with bond angles of approximately 120°.
3. Recognize that trigonal pyramidal geometry also has three atoms bonded to the central atom but includes one lone pair, which affects the shape by pushing the bonded atoms downward, creating a pyramid-like structure.
4. Note that the presence of the lone pair in trigonal pyramidal geometry reduces the bond angles from the ideal 120° seen in trigonal planar to about 107° due to lone pair-bond pair repulsion.
5. Conclude that the key difference is the presence of a lone pair on the central atom in trigonal pyramidal geometry, while trigonal planar has none, which affects both shape and bond angles.