In chemistry, valence bond theory explains the nature of a chemical bond in a molecule in terms of atomic valencies.^[1] Valence bond theory summarizes the rule that the central atom in a molecule likes to form electron pair bonds in accordance with geometric constraints as defined by the octet rule, approximately. Valence bond theory is closely related to molecular orbital theory. It has been suggested that the central science be merged into this article or section. ... A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. ... In science, a molecule is a group of atoms in a definite arrangement held together by chemical bonds. ... In chemistry, valence, also known as valency or valency number, is a measure of the number of chemical bonds formed by the atoms of a given element. ... The bonding in carbon dioxide The octet rule is a simple chemical rule of thumb that states that atoms tend to combine in such a way that they each have eight electrons in their valence shells, similar to the electronic configuration of a noble gas. ... In quantum chemistry, molecular orbitals are the statistical states electrons can have within molecules. ...

History

In 1916, G.N. Lewis proposed that a chemical bond forms by the interaction of two shared bonding electrons, with the representation of molecules as Lewis structures. In 1927 the Heitler-London theory was formulated which for the first time enabled the calculation of bonding properties of the hydrogen molecule H₂ based on quantum mechanical considerations. Specifically, after a long nap one day Walter Heitler figured out how to use Schrödinger’s wave equation (1925) to show how two hydrogen atom wavefunctions join together, with plus, minus, and exchange terms, to form a covalent bond. He then called up his associate Fritz London and they worked out the details of the theory over the course of the night.^[2] Later, Linus Pauling used the pair bonding ideas of Lewis together with Heitler-London theory to develop two other key concepts in VB theory: resonance (1928) and orbital hybridization (1930). According to Charles Coulson, author of the noted 1952 book Valance, this period marks the start of “modern valence bond theory”, as contrasted with older valence bond theories, which are essentially electronic theories of valence crouched in pre-wave-mechanical terms. Resonance theory was critizied as imperfect by Soviet chemists during the 1950ies (I. Hargittai, When Resonance Made Waves, The Chemical Intelligencer 1, 34 (1995)). Lewis in the Berkeley Lab Gilbert Newton Lewis (October 23, 1875-March 23, 1946) was a famous physical chemist. ... G. N. Lewis Lewis structures, also called electron-dot structures or electron-dot diagrams, are diagrams that show the bonding between atoms of a molecule, and the lone pairs of electrons that may exist in the molecule. ... Walter Heinrich Heitler (02. ... Fritz Wolfgang London (March 7, 1900–March 30, 1954) was a German-born American physicist for whom the London force is named. ... Walter Heinrich Heitler (02. ... The wave equation is an important partial differential equation that describes the propagation of a variety of waves, such as sound waves, light waves and water waves. ... This article discusses the concept of a wavefunction as it relates to quantum mechanics. ... Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. ... Fritz Wolfgang London (March 7, 1900–March 30, 1954) was a German-born American physicist for whom the London force is named. ... Linus Carl Pauling (February 28, 1901 – August 19, 1994) was an American quantum chemist and biochemist. ... Resonance in chemistry is a tool used to represent and model certain types of non-classical molecular structures. ... In chemistry, hybridisation is the mixing of atomic orbitals to form new orbitals suitable for bonding. ... Charles Alfred Coulson (1910-1974) was a prominent researcher in the field of theoretical chemistry. ... In chemistry, valence, also known as valency or valency number, is a measure of the number of chemical bonds formed by the atoms of a given element. ...

Theory

A valence bond structure is similar to a Lewis structure, however where a single Lewis structure cannot be written, several valence bond structures are used. Each of these VB structures represents a specific Lewis structure. This combination of valence bond structures is the main point of resonance theory. Valence bond theory considers that the overlapping atomic orbitals of the participating atoms form a chemical bond. Because of the overlapping, it is most probable that electrons should be in the bond region. Valence bond theory views bonds as weakly coupled orbitals (small overlap). Valence bond theory is typically easier to employ in ground state molecules. G. N. Lewis Lewis structures, also called electron-dot structures or electron-dot diagrams, are diagrams that show the bonding between atoms of a molecule, and the lone pairs of electrons that may exist in the molecule. ... Resonance in chemistry is a tool used to represent and model certain types of non-classical molecular structures. ... An atomic orbital is the description of the behavior of an electron in an atom according to quantum mechanics. ... A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. ... Probability is the chance that something is likely to happen or be the case. ... Properties The electron (also called negatron, commonly represented as e−) is a subatomic particle. ... In physics, the ground state of a quantum mechanical system is its lowest-energy state. ...

The overlapping atomic orbitals can differ. The two types of overlapping orbitals are sigma and pi. Sigma bonds occur when the orbitals of two shared electrons overlap head-to-head. Pi bonds occur when two orbitals overlap when they are parallel. For example, a bond between two s-orbital electrons is a sigma bond, because two spheres are always coaxial. In terms of bond order, single bonds have one sigma bond, double bonds consist of one sigma bond and one pi bond, and triple bonds contain one sigma bond and two pi bonds. However, the atomic orbitals for bonding may be hybrids. Often, the bonding atomic orbitals have a character of several possible types of orbitals. The methods to get an atomic orbital with the proper character for the bonding is called hybridization. Electron atomic and molecular orbitals, showing among others the sigma bond of two s-orbitals and a sigma bond of two p-orbitals In chemistry, sigma bonds (σ bonds) are a type of covalent chemical bond. ... Electron atomic and molecular orbitals, showing a Pi-bond at the bottom right of the picture In chemistry, pi bonds (π bonds) are bonds with a single nodal plane containing the line segment between the two atoms. ... In chemistry, hybridisation is the mixing of atomic orbitals to form new orbitals suitable for bonding. ...

VB theory today

Valence bond theory now complements Molecular Orbital Theory (MO theory), which does not adhere to the VB idea that electron pairs are localized between two specific atoms in a molecule but that they are distributed in sets of molecular orbitals which can extend over the entire molecule. MO theory can predict magnetic properties in a straight forward manner, but valence bond theory is more complicated although giving the similar results. Valence bond theory views aromatic properties of molecules as due to resonance between Kekule, Dewar and possibly ionic structures, while molecular orbital theory views it as delocalisation of the π-electrons. The underlying mathematics are also more complicated limiting VB treatment to relatively small molecules. On the other hand, VB theory provides a much more accurate picture of the reorganization of electronic charge that takes place when bonds are broken and formed during the course of a chemical reaction. In particular, valence bond theory correctly predicts the dissociation of homonuclear diatomic molecules into separate atoms, while simple molecular orbital theory predicts disociation into a mixture of atoms and ions. In chemistry, a molecular orbital is a region in which an electron may be found in a molecule. ... In chemistry, a molecular orbital is a region in which an electron may be found in a molecule. ... It has been suggested that this article or section be merged with magnet. ... Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. ... Euclid, Greek mathematician, 3rd century BC, as imagined by by Raphael in this detail from The School of Athens. ...

More recently, several groups have developed what is often called modern valence bond theory. This replaces the overlapping atomic orbitals by overlapping valence bond orbitals that are expanded over all basis functions in the molecule. The resulting energies are more competitive with energies where electron correlation is introduced based on a Hartree-Fock reference wavefunction. Modern valence bond theory has been developed by several workers, including Gerratt, Cooper and Raimondi(1997); Li and McWeeny(2002); Song, Mo, Zhang and Wu (2005); and Shaik and Hiberty (2004). ... In modern computational chemistry, quantum chemical calculations are typically performed within a finite set of basis functions. ... Electronic correlation refers to the interaction between electrons in a quantum system whose electronic structure is being considered. ... In computational physics and computational chemistry, the Hartree-Fock (HF) or self-consistent field (SCF) calculation scheme is a self-consistent iterative variational procedure to calculate the Slater determinant (or the molecular orbitals which it is made of) for which the expectation value of the electronic molecular Hamiltonian is minimum. ...

References