Advanced Placement (AP) Chemistry Practice Exam

What molecular geometry results from a molecule with three lone pairs and two bonds?

1. Linear

2. Trigonal pyramidal

3. T-shaped

4. Bent

The correct answer is: T-shaped

For a molecule with three lone pairs and two bonded atoms, the correct molecular geometry is T-shaped. This arrangement arises from the electron-pair geometry, which is based on the regions of electron density around the central atom. When considering a central atom that has a total of five electron pairs, comprising three lone pairs and two bonds, the electron-pair geometry is trigonal bipyramidal. In this geometry, the lone pairs will occupy the equatorial positions because this minimizes electron pair repulsion due to their larger spatial requirement compared to bonding pairs. With three lone pairs taking up the equatorial positions, they push the two bonding pairs into the vertical (axial) positions, leading to a T-shaped molecular geometry. Thus, T-shaped molecular geometry is the result when a central atom has three lone pairs and two bonds, as it effectively reflects the arrangement of the bonded atoms and lone pairs in three-dimensional space around the central atom.