Valence Shell Electron Pair Repulsion (VSEPR) Theory
The Valence Shell Electron Pair Repulsion (VSEPR) Theory
The Valence Shell Electron Pair Repulsion (VSEPR) Theory is a model used in chemistry to predict the molecular geometry of a molecule. It is based on the idea that electron pairs in the valence shell of a central atom arrange themselves as far apart as possible to minimize the electrostatic repulsion between them.
Developed by Ronald J. Gillespie in 1957, VSEPR theory provides a simple yet effective way to predict the shapes of molecules based on the number of electron pairs (both bonding and non-bonding) around the central atom.
Core Principles of VSEPR Theory:
- Valence Electron Pairs: The geometry of a molecule is determined by the number of electron pairs (also called electron domains or regions of electron density) in the valence shell of the central atom.
- Repulsion Minimization: These electron pairs arrange themselves in space to be as far apart as possible, thereby minimizing the repulsive forces between them. This arrangement leads to specific molecular geometries.
- Order of Repulsion: The strength of repulsion between electron pairs follows the order:
- Multiple Bonds: A double bond or a triple bond between two atoms counts as a single electron domain for the purpose of determining geometry, as the electron density is localized between the two atoms.
Lone Pair - Lone Pair (LP-LP) > Lone Pair - Bonding Pair (LP-BP) > Bonding Pair - Bonding Pair (BP-BP)
This order is important when determining deviations from ideal geometries, especially when lone pairs are present.
Steps to Determine Molecular Geometry using VSEPR Theory:
- Draw the Lewis Structure: Determine the correct Lewis structure for the molecule or polyatomic ion.
- Identify the Central Atom: Locate the central atom, which is usually the least electronegative atom (excluding Hydrogen) or the atom bonded to the most other atoms.
- Count Total Electron Domains: Count the total number of electron domains around the central atom. An electron domain can be a single bond, a double bond, a triple bond, or a lone pair of electrons.
- Determine the Electron Domain Geometry: Based on the total number of electron domains, predict the electron domain geometry (the arrangement of all electron pairs).
- 2 domains: Linear
- 3 domains: Trigonal Planar
- 4 domains: Tetrahedral
- 5 domains: Trigonal Bipyramidal
- 6 domains: Octahedral
- Determine the Molecular Geometry: Consider only the positions of the atoms (bonding pairs). The molecular geometry is the arrangement of atoms, while the electron domain geometry includes lone pairs.
Common Molecular Geometries based on Electron Domain Geometry:
The molecular geometry depends on the number of bonding pairs (BP) and lone pairs (LP) around the central atom.
| Total Electron Domains | Number of BP | Number of LP | Electron Domain Geometry | Molecular Geometry | Example | Bond Angles (Approx.) |
|---|---|---|---|---|---|---|
| 2 | 2 | 0 | Linear | Linear | $$BeCl_2$$ | 180° |
| 3 | 3 | 0 | Trigonal Planar | Trigonal Planar | $$BF_3$$ | 120° |
| 3 | 2 | 1 | Trigonal Planar | Bent (or V-shaped) | $$SO_2$$ | <120° |
| 4 | 4 | 0 | Tetrahedral | Tetrahedral | $$CH_4$$ | 109.5° |
| 4 | 3 | 1 | Tetrahedral | Trigonal Pyramidal | $$NH_3$$ | 107° |
| 4 | 2 | 2 | Tetrahedral | Bent (or V-shaped) | $$H_2O$$ | 104.5° |
| 5 | 5 | 0 | Trigonal Bipyramidal | Trigonal Bipyramidal | $$PCl_5$$ | 90°, 120°, 180° |
| 5 | 4 | 1 | Trigonal Bipyramidal | See-Saw | $$SF_4$$ | 90°, 120°, 180° (distorted) |
| 5 | 3 | 2 | Trigonal Bipyramidal | T-shaped | $$ClF_3$$ | ~90° |
| 5 | 2 | 3 | Trigonal Bipyramidal | Linear | $$XeF_2$$ | 180° |
| 6 | 6 | 0 | Octahedral | Octahedral | $$SF_6$$ | 90°, 180° |
| 6 | 5 | 1 | Octahedral | Square Pyramidal | $$BrF_5$$ | ~90° |
| 6 | 4 | 2 | Octahedral | Square Planar | $$XeF_4$$ | 90°, 180° |
Important Considerations:
- Lone Pair Effect: Lone pairs occupy more space than bonding pairs and exert stronger repulsive forces. This causes the bond angles to deviate from the ideal angles predicted by the basic electron domain geometry.
- Equatorial vs. Axial Positions: In trigonal bipyramidal geometry, lone pairs preferentially occupy equatorial positions to minimize LP-LP repulsion (which is greater than LP-Axial repulsion and Axial-Axial repulsion).
VSEPR theory is a powerful tool for predicting molecular shapes, which is crucial for understanding molecular polarity, reactivity, and intermolecular forces.