{{Short description|Theory of molecular geometry}}
== VSEPR Theory ==
{{Use dmy dates|date=October 2023}}
'''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.
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.
==History==
== Basic Principles ==
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==
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 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 main principles of VSEPR theory are:
The VSEPR model predicts several common molecular geometries:
===Linear===
1. Electron pairs repel each other and will therefore adopt an arrangement that minimizes this repulsion.
Molecules with two electron pairs around the central atom adopt a linear geometry. An example is carbon dioxide (CO_).
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.
===Trigonal Planar===
== Molecular Geometries ==
Three electron pairs around the central atom result in a trigonal planar shape, as seen in boron trifluoride (BF_).
===Tetrahedral===
=== Linear Geometry ===
Four electron pairs form a tetrahedral shape, exemplified by methane (CH_).
===Trigonal Bipyramidal===
[[File:AX2E0-2D.png|Linear geometry|thumb|right]]
Five electron pairs lead to a trigonal bipyramidal geometry, such as in phosphorus pentachloride (PCl_).
===Octahedral===
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).
Six electron pairs result in an octahedral shape, as seen in sulfur hexafluoride (SF_).
==Effect of Lone Pairs==
=== Trigonal Planar Geometry ===
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.
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==
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).
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.
=== Bent Geometry ===
[[File:AX2E1-2D.png|Bent geometry|thumb|right]]
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 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).
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.
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°.
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.
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.
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 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 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 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).
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.
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°.
