Determining the electron geometry of a molecule is the first step toward understanding its three-dimensional structure and predicting its chemical behavior. This process relies on the Valence Shell Electron Pair Repulsion (VSEPR) theory, which posits that groups of electrons repel one another and will arrange themselves in space to maximize the distance between them. Identifying this preferred arrangement around the central atom provides the fundamental shape, or electron geometry, that minimizes repulsive forces for the most stable configuration.
The Essential First Step: Drawing the Lewis Structure
Before any geometric prediction can be made, a correct Lewis structure must be drawn for the molecule or polyatomic ion. This two-dimensional diagram shows the arrangement of atoms, the covalent bonds connecting them, and any non-bonding electrons, known as lone pairs. The process begins by calculating the total number of valence electrons, adjusting for any charge in the case of an ion.
The central atom must be identified, typically the least electronegative element (excluding hydrogen, which is always terminal). Single bonds are drawn connecting the central atom to the surrounding terminal atoms. Remaining valence electrons are distributed first to the terminal atoms to satisfy the octet rule, and then any leftover electrons are placed on the central atom as lone pairs.
If the central atom lacks a full octet, lone pairs from terminal atoms are converted into double or triple bonds to complete the eight-electron requirement. This precise accounting of all bonding and non-bonding electrons is necessary because VSEPR theory considers all these electron groups when determining geometry. The completed Lewis structure provides a map of the electron density regions that will determine the final shape.
Identifying and Counting Electron Domains
The core mechanism for determining electron geometry involves identifying and counting the number of electron domains, also known as the steric number, surrounding the central atom. An electron domain is defined as any region of high electron density around the central atom, representing where electrons are most likely to be found. These domains repel one another, forcing the molecule into a specific three-dimensional arrangement.
All types of bonds—single, double, or triple—count as only one electron domain because they represent a single, localized region of electron density. For example, the carbon atom in carbon dioxide (\(\text{CO}_2\)) has two double bonds but is counted as having only two electron domains. Conversely, a lone pair of non-bonding electrons on the central atom also counts as one distinct electron domain.
In the water molecule (\(\text{H}_2\text{O}\)), the central oxygen atom has two bonding domains (single bonds) and two non-bonding domains (lone pairs), totaling four electron domains. Similarly, the nitrogen atom in ammonia (\(\text{NH}_3\)) has three bonding domains and one lone pair, resulting in a total of four electron domains. This total count is the sole determinant of the electron geometry name.
Classifying Geometries Based on Domain Count
Once the total number of electron domains is established, this number directly corresponds to one of the primary electron geometries. The geometry is named according to the spatial arrangement that places the domains farthest apart to minimize repulsion. This arrangement is fixed regardless of whether the domains are bonding pairs or lone pairs.
A central atom with two electron domains adopts a Linear geometry, orienting the domains \(180^\circ\) apart (e.g., carbon dioxide). Three domains result in a Trigonal Planar geometry, spreading out in a flat plane with ideal bond angles of \(120^\circ\) (e.g., boron trifluoride, \(\text{BF}_3\)).
Four electron domains result in a Tetrahedral geometry, a three-dimensional shape with bond angles of \(109.5^\circ\) (e.g., methane, \(\text{CH}_4\)). Five domains lead to a Trigonal Bipyramidal geometry, featuring \(90^\circ\) (axial-equatorial) and \(120^\circ\) (equatorial) bond angles. Six electron domains result in an Octahedral geometry, where all six domains are positioned at \(90^\circ\) angles relative to their neighbors.
Understanding the Difference Between Electron and Molecular Geometry
A frequent point of confusion is the distinction between electron geometry and molecular geometry, which are not always the same. Electron geometry describes the arrangement of all electron pairs—both bonding and lone pairs—around the central atom, based on the total domain count.
Molecular geometry, in contrast, describes the physical shape defined only by the positions of the atoms. Lone pairs are ignored when naming the shape, though they still influence atomic positions. When a central atom has no lone pairs (e.g., methane, \(\text{CH}_4\)), the electron geometry (tetrahedral) and the molecular geometry are identical.
If one or more lone pairs are present, the two geometries differ. For instance, ammonia (\(\text{NH}_3\)) has a tetrahedral electron geometry (four domains). Since one domain is a lone pair, the molecular geometry is Trigonal Pyramidal. Lone pairs exert a greater repulsive force than bonding pairs, compressing the actual bond angles between the atoms.