VSEPR theory: Difference between revisions

Latest revision as of 18:53, 23 March 2025

VSEPR Theory[edit]

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[edit]

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[edit]

Linear Geometry[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

Water Molecule[edit]

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[edit]

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[edit]

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-'''Valence Shell Electron Pair Repulsion (VSEPR) theory''' is a model used in [[chemistry]] to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is based on the principle that electron pairs around a central atom will organize themselves in such a way as to minimize repulsion, leading to specific, predictable molecular shapes.
+== VSEPR Theory ==
-==Overview==
+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.
-The VSEPR theory is founded on the idea that electron pairs in the valence shell of an atom repel each other since they are negatively charged. These electron pairs can be bonding pairs, which are shared between atoms in a molecule, or lone pairs, which are not shared and belong to a single atom. The spatial arrangement of these electron pairs is what determines the geometry of the molecule.
-==Predicting Molecular Geometry==
+== Basic Principles ==
-To predict the molecular geometry of a molecule using the VSEPR theory, one must first determine the Lewis structure of the molecule to identify the number of bonding pairs and lone pairs of electrons on the central atom. Then, using the VSEPR model, the shape of the molecule can be predicted based on the total number of electron pairs.
-The basic geometries for molecules based on the number of electron pairs are:
+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.
-* Linear - with 2 electron pairs
-* Trigonal planar - with 3 electron pairs
-* Tetrahedral - with 4 electron pairs
-* Trigonal bipyramidal - with 5 electron pairs
-* Octahedral - with 6 electron pairs
-Lone pairs of electrons occupy more space than bonding pairs, and this must be taken into account when predicting molecular shapes. For example, if there are lone pairs, the molecular geometry might be different from the electron pair geometry.
+The main principles of VSEPR theory are:
-==Limitations of VSEPR Theory==
+. Electron pairs repel each other and will therefore adopt an arrangement that minimizes this repulsion.
-While the VSEPR theory is useful for predicting the general shape of molecules, it has its limitations. It does not accurately predict the bond angles in molecules as precisely as other methods, such as [[Molecular Orbital Theory]]. Additionally, it cannot predict the relative sizes of molecules or the presence of multiple bonds.
+. Lone pairs exert more repulsion than bonding pairs, which affects the geometry of the molecule.
+. The geometry of the molecule is determined by the number of bonding pairs and lone pairs around the central atom.
-==Applications of VSEPR Theory==
+== Molecular Geometries ==
-Despite its limitations, the VSEPR theory is widely used in chemistry for understanding and predicting the structure of molecules. It is particularly useful in organic chemistry, where the shape of a molecule can determine its reactivity and properties. The theory also aids in the understanding of the physical properties of molecules, such as boiling points, melting points, and solubility.
+=== Linear Geometry ===
+[[File:AX2E0-2D.png|Linear geometry|thumb|right]]
+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 ===
+[[File:AX3E0-side-2D.png|Trigonal planar geometry|thumb|left]]
+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 ===
+[[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 ===
+[[File:AX4E0-2D.png|Tetrahedral geometry|thumb|left]]
+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 ===
+[[File:AX3E1-2D.png|Trigonal pyramidal geometry|thumb|right]]
+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 ===
+[[File:Water-dimensions-from-Greenwood&Earnshaw-2D.png|Water molecule dimensions|thumb|left]]
+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 ===
+[[File:Sulfur-tetrafluoride-2D-dimensions.png|Sulfur tetrafluoride dimensions|thumb|right]]
+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 ==
-==See Also==
 * [[Molecular geometry]]
-* [[Lewis structure]]
+* [[Chemical bonding]]
-* [[Molecular Orbital Theory]]
+* [[Electron pair]]
-* [[Chemical bond]]
+* [[Lone pair]]
-[[Category:Chemistry]]
+{{Chemistry}}
-[[Category:Physical chemistry]]
+[[Category:Chemical bonding]]
 [[Category:Molecular geometry]]
-{{Chem-stub}}