Advanced Placement (AP) Chemistry Practice Exam

Which molecular geometry is associated with a molecule having one lone pair and three bonds?

1. Bent

2. Trigonal pyramidal

3. Linear

4. Trigonal planar

The correct answer is: Trigonal pyramidal

The molecular geometry of a molecule that has one lone pair and three bonding pairs is trigonal pyramidal. In this arrangement, the central atom has four regions of electron density—three for the bonds and one for the lone pair. According to VSEPR (Valence Shell Electron Pair Repulsion) theory, these regions arrange themselves to minimize repulsion, leading to a geometry that resembles a pyramid with a triangular base. The lone pair occupies one of the positions, which pushes the bonded atoms down, resulting in a shape that has a bond angle of less than 109.5°, typical for a tetrahedral electron arrangement. This is characteristic of molecules like ammonia (NH₃), which showcases this geometry clearly. In contrast, a molecule with a bent geometry typically has two lone pairs and two bonds or one lone pair and two bonds when considering the bent structure of water (H₂O) which affects bond angles differently. Linear geometry would apply to a molecule with two bonding pairs and no lone pairs, and trigonal planar geometry pertains to a situation with no lone pairs and three bonded atoms. Thus, trigonal pyramidal is the fitting description for a molecule with one lone pair and three bonds.