Electron geometry describes the spatial arrangement of all electron groups, both bonding and non-bonding, around a molecule’s central atom. This organization is dictated by Valence Shell Electron Pair Repulsion theory (VSEPR). The core principle of VSEPR is that negatively charged electron clouds repel one another and arrange themselves to maximize the distance between them.
This arrangement represents the lowest-energy conformation where repulsive forces are minimized. Determining this geometry is the first step toward understanding a molecule’s full three-dimensional structure, which influences its chemical reactivity and physical properties.
Identifying the Central Atom and Valence Electrons
Finding a molecule’s electron geometry begins with constructing its Lewis structure. First, calculate the total number of valence electrons, as these are the electrons involved in bonding and repulsion. Sum the valence electrons for every atom present, using the atom’s group number on the periodic table.
If the molecule is a polyatomic ion, incorporate the charge into the total count. A negative charge means adding electrons, while a positive charge means subtracting them. For example, the carbonate ion (\(\text{CO}_3^{2-}\)) requires adding two electrons to the sum of the valence electrons.
Next, identify the central atom, which acts as the hub for the electron groups. The central atom is usually the least electronegative element. Atoms like hydrogen and halogens are rarely central because they typically form only a single bond, placing them on the periphery. If there is ambiguity, the atom capable of forming the greatest number of bonds, or the atom present in the lowest quantity, is usually central.
Calculating the Total Electron Domains
The concept of an “electron domain” represents any region of high electron density around the central atom. The total count of these domains, often called the steric number, is the sole factor determining the electron geometry. Domains are categorized as either bonding or non-bonding.
A bonding domain is created by shared electrons between the central atom and a surrounding atom. Crucially, a single, double, or triple bond all count as only one electron domain because the electrons are confined to a single, localized region of space.
A non-bonding domain is a lone pair of electrons residing on the central atom. Each lone pair counts as one distinct electron domain. The total number of domains is calculated by adding the number of atoms bonded to the central atom to the number of lone pairs on the central atom.
For example, carbon dioxide (\(\text{CO}_2\)) has two double bonds and no lone pairs, resulting in two electron domains. Conversely, the oxygen atom in water (\(\text{H}_2\text{O}\)) has two single bonds and two lone pairs, totaling four electron domains.
Mapping Domain Counts to Geometry Names
The final geometry name is directly linked to the total number of electron domains calculated around the central atom.
Two Domains: Linear
With two electron domains, the regions of electron density orient themselves 180 degrees apart, resulting in a linear electron geometry. Carbon dioxide (\(\text{CO}_2\)) exhibits this arrangement, with its two double bonds pointing in opposite directions.
Three Domains: Trigonal Planar
An atom with three electron domains adopts a trigonal planar geometry. The domains spread out in a flat plane at 120-degree angles. Boron trifluoride (\(\text{BF}_3\)) is a clear example of this shape, representing the maximum separation for three points around a center.
Four Domains: Tetrahedral
When four electron domains surround the central atom, the geometry shifts into three dimensions to become tetrahedral. This shape positions the domains at bond angles of approximately 109.5 degrees. Methane (\(\text{CH}_4\)) is the classic example.
Five Domains: Trigonal Bipyramidal
If the central atom has five electron domains, the arrangement is called trigonal bipyramidal. This geometry combines a triangular plane (equatorial positions) and two axial positions. It features two distinct bond angles: 120 degrees (equatorial) and 90 degrees (axial-equatorial). Phosphorus pentachloride (\(\text{PCl}_5\)) demonstrates this configuration.
Six Domains: Octahedral
Six electron domains result in an octahedral electron geometry. The domains are positioned at 90-degree angles relative to one another. This highly symmetric shape allows the six electron groups to achieve the greatest possible separation. Sulfur hexafluoride (\(\text{SF}_6\)) is the typical example.
Electron Versus Molecular Geometry
Understanding electron geometry is a necessary prerequisite for determining molecular geometry, but the two are not always the same. Electron geometry is defined by the arrangement of all electron domains (bonding and lone pairs) around the central atom. Molecular geometry, in contrast, describes the arrangement of only the atoms in the molecule, ignoring the spatial position of lone pairs.
The presence of lone pairs causes these two geometries to differ. Lone pairs exert a stronger repulsive force on surrounding bonding domains, which compresses the bond angles and distorts the final molecular shape.
Ammonia (\(\text{NH}_3\)) provides a clear example. It has four total electron domains (three bonds, one lone pair), making its electron geometry tetrahedral. However, its molecular geometry is trigonal pyramidal because the lone pair pushes the hydrogen atoms downward. Similarly, water (\(\text{H}_2\text{O}\)) has a tetrahedral electron geometry but a bent molecular geometry due to two lone pairs.
When a central atom has no lone pairs, such as in methane (\(\text{CH}_4\)), the electron geometry and the molecular geometry are identical (tetrahedral).