VSEPR theory

VSEPR Theory

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

Basic Principles

VSEPR theory postulates that the shape of a molecule is determined by the repulsions between all of the electron pairs present in the valence shell of the central atom. These electron pairs can be either bonding pairs, which are shared between atoms, or lone pairs, which are not shared and belong to a single atom.

The main principles of VSEPR theory are:

1. Electron pairs repel each other and will therefore adopt an arrangement that minimizes this repulsion. 2. Lone pairs exert more repulsion than bonding pairs, which affects the geometry of the molecule. 3. The geometry of the molecule is determined by the number of bonding pairs and lone pairs around the central atom.

Molecular Geometries

Linear Geometry

Linear geometry

In linear geometry, the central atom is surrounded by two regions of electron density, which are arranged 180° apart. This geometry is typical for molecules with the formula AX₂, such as carbon dioxide (CO₂).

Trigonal Planar Geometry

Trigonal planar geometry

Trigonal planar geometry occurs when there are three regions of electron density around the central atom, arranged 120° apart. This geometry is seen in molecules with the formula AX₃, such as boron trifluoride (BF₃).

Bent Geometry

Bent geometry

Bent geometry is observed when there are two bonding pairs and one or more lone pairs on the central atom. The presence of lone pairs causes the bond angle to be less than 120°. Water (H₂O) is a common example, with a bond angle of approximately 104.5°.

Tetrahedral Geometry

Tetrahedral geometry

Tetrahedral geometry is characterized by four regions of electron density around the central atom, arranged at angles of 109.5°. This geometry is typical for molecules with the formula AX₄, such as methane (CH₄).

Trigonal Pyramidal Geometry

Trigonal pyramidal geometry

In trigonal pyramidal geometry, there are three bonding pairs and one lone pair around the central atom. The lone pair causes the bond angles to be slightly less than 109.5°. Ammonia (NH₃) is a classic example of this geometry.

Examples of Molecular Geometries

Water Molecule

Water molecule dimensions

The water molecule (H₂O) has a bent geometry due to the two lone pairs on the oxygen atom, which repel the hydrogen atoms and reduce the bond angle to about 104.5°.

Sulfur Tetrafluoride

Sulfur tetrafluoride dimensions

Sulfur tetrafluoride (SF₄) has a seesaw shape due to the presence of one lone pair on the sulfur atom, which distorts the ideal tetrahedral geometry.

Related Pages

• Molecular geometry
• Chemical bonding
• Electron pair
• Lone pair