The Valence Shell Electron Pair Repulsion (VSEPR) theory is a straightforward model used in chemistry to predict the three-dimensional shape of molecules. This arrangement, known as molecular geometry, is a fundamental property because it dictates a molecule’s chemical reactivity, polarity, and physical characteristics like boiling point. VSEPR helps visualize how atoms are spatially organized around a central atom in a compound.
The Core Principle: Minimizing Electron Repulsion
The shapes that molecules adopt are driven by a simple, universal force: the repulsion between negative charges. Valence electrons, which are the electrons in the outermost shell of an atom, exist as pairs—either shared in a bond or as unshared lone pairs. Because all electrons possess a negative charge, these electron pairs naturally repel one another, seeking to maximize the distance between them in three-dimensional space.
The geometry a molecule assumes is the one that achieves the lowest energy state by minimizing this inter-electron repulsion. This optimal arrangement places the electron pairs as far apart as possible around the central atom. Lone pairs, which are held closer to a single nucleus, exert a greater repulsive force than bonding pairs, which are shared between two nuclei. This difference in repulsive strength significantly influences the final bond angles and overall molecular shape.
Applying the Theory: Counting Electron Domains
To use VSEPR theory, the first step is to count the number of “electron domains” surrounding the central atom. An electron domain is a region of space where electrons are likely to be found, and this counting process treats all groups of electrons as equivalent repulsive units. The number of domains determines the initial, idealized geometric arrangement for all electron groups.
For counting purposes, a single bond, a double bond, and a triple bond each count as just one electron domain. This is because, regardless of the number of electron pairs shared, they all occupy the same single region of space between the two bonded atomic nuclei. Furthermore, every unshared lone pair of electrons on the central atom also counts as one electron domain.
Consider the methane molecule (\(\text{CH}_4\)), where the central carbon atom forms four single bonds with hydrogen atoms; this results in four electron domains. In contrast, the carbon in carbon dioxide (\(\text{CO}_2\)) forms two double bonds, which only count as two electron domains. Ammonia (\(\text{NH}_3\)) has three single bonds and one lone pair on the central nitrogen atom, totaling four electron domains. Accurately counting these domains is the groundwork for predicting the final molecular structure.
Translating Domains into Molecular Shapes
The total number of electron domains determines the electron geometry, which is the arrangement of all electron groups (both bonding and lone pairs) around the central atom.
The electron geometries corresponding to the number of domains are:
- Two domains: Linear
- Three domains: Trigonal planar
- Four domains: Tetrahedral
- Five domains: Trigonal bipyramidal
- Six domains: Octahedral
The final molecular geometry describes the arrangement of only the atoms in the molecule, which is determined by the positions of the bonding domains. This is where the influence of lone pairs becomes apparent, as they change the molecular shape without changing the electron geometry. For example, molecules with four electron domains all have a tetrahedral electron geometry.
Methane (\(\text{CH}_4\)), with four bonding domains and zero lone pairs, maintains a tetrahedral molecular geometry. However, ammonia (\(\text{NH}_3\)) has three bonding domains and one lone pair; the atoms arrange in a trigonal pyramidal shape, as the lone pair pushes the three hydrogen atoms downward. The water molecule (\(\text{H}_2\text{O}\)) has two bonding domains and two lone pairs, resulting in a bent molecular geometry. The stronger repulsion exerted by the lone pairs in water also compresses the bond angle from the ideal tetrahedral angle of \(109.5^\circ\) to approximately \(104.5^\circ\).