Valence Shell Electron Pair Repulsion Theory
The Valence Shell Electron Pair Repulsion (VSEPR) theory is a straightforward model used by chemists to predict the three-dimensional geometric shape of molecules. The fundamental premise is that electron pairs, all being negatively charged, will arrange themselves in space to achieve the maximum possible separation from one another. This spatial arrangement minimizes the electrostatic repulsion between them, leading to the most energetically stable molecular structure.
The Fundamental Concept of Electron Domains
VSEPR relies on the concept of an “electron domain,” which is a region of high electron density around the central atom. An electron domain can be a single bond, a double bond, a triple bond, or a lone pair of non-bonding electrons. Crucially, a multiple bond, like a double or triple bond, counts as only one electron domain because the electrons involved are all confined to the same general region of space between the two atoms.
These domains contain negatively charged electrons and thus repel every other domain surrounding the central atom. This repulsion is the driving force that determines the molecule’s geometry, causing the domains to spread out as far away from each other as possible. The resulting optimal arrangement places the domains at specific angles that minimize the overall repulsive energy of the system.
A key distinction exists between bonding domains, which are shared electron pairs forming a covalent bond, and non-bonding domains, which are lone pairs belonging exclusively to the central atom. Non-bonding domains, or lone pairs, tend to occupy more space than bonding domains because they are held closer to the central atom’s nucleus and are not simultaneously attracted to a second nucleus. This difference in spatial requirement means that repulsions involving lone pairs are stronger than those between bonding pairs, which influences the final molecular shape.
Systematic Steps for Determining Electron Arrangement
The first prerequisite step is to accurately determine and draw the molecule’s Lewis structure, which visually represents all valence electrons, including both bonding and lone pairs. This structure identifies the central atom and shows the distribution of all electron pairs around it.
Once the Lewis structure is established, the next action is to count the total number of electron domains surrounding the central atom. This count includes every single bond, every multiple bond (each counting as one domain), and every lone pair of electrons on the central atom. This total number of domains, often called the steric number, dictates the overall spatial orientation of the electron groups.
For instance, a central atom with four electron domains will always adopt a tetrahedral arrangement in space to maximize the separation between the domains. The domains arrange themselves at specific angles (like \(180^\circ\) for two domains or \(120^\circ\) for three domains) to achieve this minimum-repulsion configuration.
Translating Electron Arrangement into Molecular Shape
The total number of electron domains determines the molecule’s electron geometry, which describes the arrangement of all electron groups around the central atom, including both bonding and non-bonding pairs. For two, three, four, five, and six domains, the electron geometries are consistently linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral, respectively.
The molecular geometry, however, describes the arrangement of only the atoms in the molecule, excluding the lone pairs. When all electron domains are bonding pairs, the electron geometry and the molecular geometry are identical, such as in methane (\(\text{CH}_4\)), which has four bonding domains and a tetrahedral shape.
Lone pairs still occupy space and exert repulsive forces, but since they are not visible as bonded atoms, they alter the final observed shape. For example, both ammonia (\(\text{NH}_3\)) and water (\(\text{H}_2\text{O}\)) start with a tetrahedral electron geometry because they each have four total domains. Ammonia has three bonding pairs and one lone pair, which pushes the three hydrogen atoms into a trigonal pyramidal molecular shape. Water has two bonding pairs and two lone pairs, causing even greater compression of the bond angle and resulting in a bent or V-shaped molecular geometry. The greater repulsive force of the lone pairs compresses the angle between the bonded atoms, causing a deviation from the ideal \(109.5^\circ\) tetrahedral angle to approximately \(107^\circ\) in ammonia and \(104.5^\circ\) in water.