The valence shell electron pair repulsion theory or VSEPR is a model in chemistry that aims to generally represent the shapes of individual molecules. To achieve this, it is necessary to construct a valid Lewis structure that shows all of the bonds within the molecule and the locations of lone pairs of electrons. To predict the molecular geometry, the total coordination number of the central atom is taken into account. A model is a substitute for or an abstraction of a specific thing. ... Chemistry (in Greek: χημεία) is the science of matter and its interactions with energy and itself (see physics, biology). ... Geometry of the water molecule Molecules have fixed equilibrium geometries--bond lengths and angles--that are dictated by the laws of quantum mechanics. ... A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ... A diagram of several simple Lewis structures of molecules and atoms. ... Geometry of the water molecule Molecules have fixed equilibrium geometries--bond lengths and angles--that are dictated by the laws of quantum mechanics. ... In chemistry, the coordination number is the sum of the total number of neighbours of a central atom in a chemical compound and the number of lone pairs on it. ...

VSEPR theory is based on the idea that the geometry of a molecule or polyatomic ion is determined primarily by repulsion among the pairs of electrons associated with a central atom. The pairs of electrons may be bonding or nonbonding (also called lone pairs). Only electrons in the valence of the central atom influence the molecular shape in a meaningful way. Repulsion is a film made in 1965 directed by Roman Polanski. ... Valence is a scientific term in chemistry to describe electrons in the outermost orbital. ...

Three types of repulsion take place between the electrons of a molecule:

• The lone pair-lone pair repulsion
• The lone pair-bonding pair repulsion between the atoms
• The bonding pair-bonding pair repulsion.

A molecule must avoid these repulsions to remain stable. The theory states that the repulsion becomes zero at ~115-120°. When repusion cannot be avoided, the weaker repulsion (i.e. the one that causes the smallest deviation from the ideal shape) is preferred.

The lone pair-lone pair (lp-lp) repulsion is considered to be stronger than the lone pair-bonding pair (lp-bp) repulsion, which in turn is stronger than the bonding pair-bonding pair (bp-bp) repulsion. Hence, the weaker bp-bp repulsion is preffered over the lp-lp or lp-bp repulsion.

Larger molecules which fail to even maintain 90° between their electron pairs prefer to lie in more than one plane.

VSEPR theory is usually compared and contrasted with valence bond theory, which addresses molecular shape through orbitals that are energentically accessible for bonding. Valence bond theory concerns itself with the formation of sigma and pi bonds. Molecular orbital theory is a more sophisticated model for understanding how atoms and electrons are assembled into molecules and polyatomic ions. The valence bond theory considers the overlap of the atomic orbitals of the participation atoms to form a chemical bond. ... In quantum chemistry, molecular orbitals are the statistical states electrons can have within molecules. ...

The AXE method is commonly used in formatting molecules to fit the VSEPR model. In chemistry, The AXE method is commonly used in formatting molecules to fit the VSEPR model that aims to explain molecular geometry. ...

Examples

The methane molecule (CH₄) is tetrahedral because there are four pairs of electrons. The four hydrogen atoms are positioned at the vertices of a tetrahedron, and the bond angle is 109.5°. This is referred to as an AB₄ type of molecule. A is the central atom and B represents all of the outer atoms. The simplest hydrocarbon, methane, is a gas with a chemical formula of CH4. ... A tetrahedron (plural: tetrahedra) is a polyhedron composed of four triangular faces, three of which meet at each vertex. ...

The ammonia molecule (NH₃) has three pairs of electrons involved in bonding, but there is a lone pair of electrons on the nitrogen atom. It is not bonded with another atom; however, it influences the overall shape through repulsions. As in methane above, there are four regions of electron density. Therefore, the overall orientation of the regions of electron density is tetrahedral. On the other hand, there are only three outer atoms. This is referred to as an AB₃E type molecule because the lone pair is represented by an E. The overall shape of the molecule is a trigonal pyramid because the lone pair is not "visible." The shape of a molecule is found from the relationship of the atoms even though it can be influenced by lone pairs of electrons. Ammonia is a chemical compound with the formula NH3. ...

Image File history File links Description: Shows the geometries of molecular shapes based on steric numbers and # of lone pairs. ...

See also

• Molecular geometry
• Linear combination of atomic orbitals
• AXE method (chemistry)