The three-dimensional shape of a molecule dictates its physical and chemical properties. Electron Domain Geometry (EDG) is a fundamental concept used to predict this shape, referring to the spatial arrangement of electron groups—both bonding and non-bonding—around a central atom. This predictive process is rooted in the Valence Shell Electron Pair Repulsion (VSEPR) theory, which posits that electron domains repel one another and arrange themselves to maximize the distance between them, minimizing repulsive forces. Determining the EDG establishes the foundational geometry from which the final atomic shape is derived.
Establishing the Central Atom and Lewis Structure
Accurately determining the Electron Domain Geometry begins with constructing a correct Lewis structure for the molecule or ion. The central atom is typically the least electronegative element in the compound, excluding hydrogen, as hydrogen can only form a single bond and cannot be central. In cases where atoms have similar electronegativity, the atom that can form the most bonds or is the single atom in the formula is usually the central one. For example, in water (\(\text{H}_2\text{O}\)), oxygen is the central atom.
Once the central atom is identified, calculate the total number of valence electrons, adjusting for any positive or negative charge on an ion. These electrons form single bonds between the central atom and surrounding atoms. The remaining electrons are distributed as lone pairs, first to satisfy the octets of the surrounding atoms, and then to the central atom. The Lewis structure visually represents all the electron groups that define the electron domains around the central atom.
Counting the Total Number of Electron Domains
The Electron Domain Geometry depends on the total number of electron domains surrounding the central atom. An electron domain is defined as any region of electron density around the central atom. A domain can be a single bond, a double bond, a triple bond, or a lone pair of electrons.
Each non-bonding lone pair on the central atom counts as one electron domain. Any bond to a surrounding atom, regardless of its type, also counts as only one electron domain. For example, in carbon dioxide (\(\text{CO}_2\)), the central carbon atom is double-bonded to two oxygen atoms, 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. This counting method, often referred to as the steric number, is the direct input for determining the molecule’s fundamental geometry.
Assigning the Electron Domain Geometry Name
The total number of electron domains correlates directly to the Electron Domain Geometry, which is the specific geometric arrangement that minimizes repulsion according to VSEPR theory.
- Two domains result in a Linear arrangement (\(180^{\circ}\)).
- Three domains result in a Trigonal Planar arrangement (\(120^{\circ}\)).
- Four domains result in a Tetrahedral arrangement (\(109.5^{\circ}\)).
- Five domains result in a Trigonal Bipyramidal arrangement.
- Six domains result in an Octahedral arrangement.
The Trigonal Bipyramidal shape has both axial and equatorial positions. The Octahedral shape positions the six domains at \(90^{\circ}\) angles to each other. This assigned name describes the shape of the entire electron cloud.
The Critical Distinction Between Electron Domain and Molecular Geometry
The distinction between Electron Domain Geometry (EDG) and Molecular Geometry is important. EDG accounts for the spatial arrangement of all electron domains, including lone pairs and bonding pairs. Molecular Geometry, however, describes the arrangement of only the atoms in three-dimensional space, effectively ignoring the lone pairs when naming the shape, though not their influence.
For example, methane (\(\text{CH}_4\)), ammonia (\(\text{NH}_3\)), and water (\(\text{H}_2\text{O}\)) all have four electron domains around their central atoms, meaning their EDG is Tetrahedral. Their molecular geometries differ because of the varying number of lone pairs.
Methane (no lone pairs) is Tetrahedral. Ammonia (one lone pair) is Trigonal Pyramidal. Water (two lone pairs) is Bent. The lone pairs occupy space and exert a greater repulsive force than bonding pairs, which pushes the bonded atoms closer together, distorting the final visible shape of the molecule.