The Valence Shell Electron Pair Repulsion (VSEPR) theory is a straightforward but powerful model chemists use to predict the three-dimensional arrangement of atoms within a molecule. This spatial organization, known as molecular geometry, is determined by the positioning of the valence electrons around a central atom. The theory operates on the principle that the electron pairs in the outermost shell of an atom will arrange themselves to minimize the repulsive forces acting between them.
The Core Principle: Electron Domain Repulsion
The direct answer to what is repelling what in VSEPR theory involves the concept of an “electron domain.” An electron domain is simply a region of electron density around a central atom, which can be a single bond, a double bond, a triple bond, or a lone pair of electrons. The theory posits that these electron domains, all being composed of negatively charged electrons, naturally repel one another. The fundamental goal of the central atom is to position these domains as far apart from each other as possible in three-dimensional space. This arrangement maximizes the distance between the negative charges, thereby achieving the lowest possible energy state.
Whether the electrons are shared in a bond (bonding pairs) or belong solely to the central atom (lone pairs), the repulsive force they exert drives the geometry of the molecule. For instance, if a central atom has four electron domains, the only way to maximize the separation between them is by positioning them at the corners of a tetrahedron. This initial geometric arrangement of the electron domains is called the electron geometry.
Hierarchy of Repulsive Forces
While all electron domains repel each other, they do not all exert the same strength of repulsion. The VSEPR model refines its predictions by establishing a hierarchy of repulsive forces. This distinction is made between bonding pairs (BP) of electrons, which are shared between two atoms, and lone pairs (LP), which belong only to the central atom. Lone pairs of electrons exert a stronger repulsive force than bonding pairs because they are held closer to the central atom’s nucleus and occupy a larger volume of space. A bonding pair is stretched out between two atomic nuclei, making it less concentrated and less repulsive.
This difference in spatial occupation leads to a defined order of repulsive strength: Lone Pair–Lone Pair (LP-LP) repulsion is the strongest, followed by Lone Pair–Bonding Pair (LP-BP) repulsion, and finally, Bonding Pair–Bonding Pair (BP-BP) repulsion, which is the weakest. This established hierarchy (LP-LP > LP-BP > BP-BP) is what causes deviations from the ideal bond angles predicted by the electron geometry. The stronger LP-LP and LP-BP repulsions push the adjacent bonding pairs closer together, compressing the angle between them.
Predicting Molecular Shapes
The application of the VSEPR theory involves two main steps: determining the electron geometry and then establishing the final molecular shape. The electron geometry is set by the total number of electron domains, which arrange themselves to achieve maximum separation. The molecular shape, however, is determined only by the positions of the atoms bonded to the central atom, which is where the repulsive hierarchy plays its role.
Consider Methane (\(CH_4\)), which has four bonding pairs and zero lone pairs around the central carbon atom. Since all four domains are bonding pairs, the repulsions are all of the weakest BP-BP type, resulting in a perfectly symmetrical tetrahedral electron and molecular geometry with ideal \(109.5^{\circ}\) bond angles.
Ammonia (\(NH_3\)) presents a different case, with three bonding pairs and one lone pair around the nitrogen atom, totaling four electron domains. The electron geometry remains tetrahedral, but the lone pair exerts a stronger LP-BP repulsion on the three bonding pairs. This stronger push compresses the \(H-N-H\) bond angle from the ideal \(109.5^{\circ}\) to approximately \(107^{\circ}\), resulting in a trigonal pyramidal molecular shape. Water (\(H_2O\)) provides an example with two bonding pairs and two lone pairs around the central oxygen atom, still totaling four electron domains. The two lone pairs exert the strongest LP-LP repulsion on each other and a strong LP-BP repulsion on the bonding pairs, further reducing the \(H-O-H\) bond angle to about \(104.5^{\circ}\), creating a bent or angular molecular shape.