Electron domain geometry (EDG) is a concept used in chemistry to predict the three-dimensional shape of molecules. This prediction is based on the Valence Shell Electron Pair Repulsion (VSEPR) theory. VSEPR posits that negatively charged electron groups around a central atom naturally repel each other. To minimize this repulsion and achieve the most stable state, these electron groups position themselves as far apart as possible in space. The resulting spatial arrangement of all electron groups—including both bonding pairs and lone pairs—defines the molecule’s electron domain geometry.
Defining and Counting Electron Domains
An electron domain represents a distinct region of electron density around the central atom of a molecule. The counting process follows a simple rule: every single bond, double bond, triple bond, and lone pair of electrons on the central atom is counted as one electron domain. A multiple bond, such as a double or triple bond, occupies only one region of space and therefore counts as a single domain. For example, carbon dioxide (\(\mathrm{CO}_2\)) has two double bonds, counted as two electron domains. Methane (\(\mathrm{CH}_4\)), with four single bonds, has four electron domains. This total number of domains dictates the fundamental geometric framework the molecule will adopt.
The Standard Electron Domain Geometries
The total number of electron domains around the central atom corresponds directly to one of the five fundamental electron domain geometries. This geometry describes the arrangement that maximizes the distance between the electron groups. Two electron domains form a Linear geometry with a bond angle of \(180^\circ\). Three domains result in a Trigonal Planar geometry, where the domains lie in the same plane separated by ideal angles of \(120^\circ\). Four domains lead to the Tetrahedral geometry, a three-dimensional shape with bond angles of approximately \(109.5^\circ\).
Molecules with five electron domains adopt a Trigonal Bipyramidal geometry, which involves two distinct types of positions. Three domains lie in an equatorial plane with \(120^\circ\) angles, while the remaining two domains are positioned axially, perpendicular to the plane. The bond angles are \(120^\circ\) within the equatorial plane and \(90^\circ\) between the axial and equatorial positions. Six electron domains arrange themselves into an Octahedral geometry. In this highly symmetric shape, all six electron domains are equivalent and form \(90^\circ\) angles with their nearest neighbors.
Electron Domain Geometry Versus Molecular Geometry
The distinction between electron domain geometry (EDG) and molecular geometry (MG) is fundamental to molecular structure. EDG accounts for the spatial arrangement of all electron groups, including both bonding pairs and lone pairs. Molecular geometry, however, describes the arrangement of the atoms only, ignoring the lone pairs when naming the final shape. Lone pairs occupy an electron domain and contribute to the EDG, but they do not define the molecular shape.
When all electron domains are bonding pairs, the EDG and the MG share the same name, such as the tetrahedral geometry of methane. The shape changes when one or more domains are occupied by lone pairs. For example, both ammonia (\(\mathrm{NH}_3\)) and water (\(\mathrm{H}_2\mathrm{O}\)) have four electron domains, resulting in a tetrahedral EDG. Ammonia has one lone pair, leading to a Trigonal Pyramidal molecular geometry, while water has two lone pairs, resulting in a Bent molecular geometry. Lone pairs influence the final arrangement by repelling bonding pairs more strongly, which reduces the bond angles from the ideal \(109.5^\circ\) tetrahedral value.