VSEPR theory

Theory of molecular geometry

VSEPR theory (Valence Shell Electron Pair Repulsion theory) is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. The theory is based on the idea that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion.

History

The VSEPR model was first proposed by Ronald Gillespie and Ronald Nyholm in 1957. It was developed to explain the shapes of molecules and ions that could not be described by the Lewis structure model alone.

Basic Principles

The VSEPR theory is based on the following principles:

• Electron pairs, both bonding and non-bonding, repel each other.
• The shape of a molecule is determined by the number of electron pairs around the central atom.
• Lone pairs occupy more space than bonding pairs, leading to distortions in molecular geometry.

Molecular Geometries

The VSEPR model predicts several common molecular geometries:

Linear

Molecules with two electron pairs around the central atom adopt a linear geometry. An example is carbon dioxide (CO_).

Trigonal Planar

Three electron pairs around the central atom result in a trigonal planar shape, as seen in boron trifluoride (BF_).

Tetrahedral

Four electron pairs form a tetrahedral shape, exemplified by methane (CH_).

Trigonal Bipyramidal

Five electron pairs lead to a trigonal bipyramidal geometry, such as in phosphorus pentachloride (PCl_).

Octahedral

Six electron pairs result in an octahedral shape, as seen in sulfur hexafluoride (SF_).

Effect of Lone Pairs

Lone pairs of electrons occupy more space than bonding pairs, causing deviations from ideal geometries. For example, in water (H_O), the two lone pairs on oxygen result in a bent shape rather than a linear one.

Applications

VSEPR theory is widely used in inorganic chemistry and organic chemistry to predict the shapes of molecules and ions. It is particularly useful for understanding the geometry of transition metal complexes.

Limitations

While VSEPR theory is useful for predicting molecular shapes, it does not account for the effects of electronegativity or the presence of multiple bonds. It also does not explain the relative strengths of different types of repulsions.

Related pages

• Molecular geometry
• Chemical bonding
• Lewis structure

References

• Gillespie, R. J., & Nyholm, R. S. (1957). Inorganic stereochemistry. Quarterly Reviews, Chemical Society, 11(3), 339-380.
• Greenwood, N. N., & Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann.

Gallery

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