American Chemical Society (ACS) Chemistry Practice Exam 2025 - Free ACS Chemistry Practice Questions and Study Guide.

The electron domain geometry refers to the spatial arrangement of all electron domains (which can be bonding or lone pairs) around a central atom. For a molecule with three electron domains, the arrangement is trigonal planar.

In a trigonal planar geometry, the three regions of electron density are arranged in a plane at 120-degree angles to minimize repulsion according to VSEPR (Valence Shell Electron Pair Repulsion) theory. This configuration allows the central atom to effectively accommodate three bonding pairs or a combination of bonding pairs and lone pairs in a planar arrangement.

In contrast, if there were two electron domains, the geometry would be linear, while four electron domains would lead to a tetrahedral arrangement. An octahedral geometry applies when there are six electron domains. Thus, only the trigonal planar geometry is compatible with having exactly three electron domains.