valence bond theory

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.

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 criticized as imperfect by Soviet chemists during the 1950's (I. Hargittai, When Resonance Made Waves, The Chemical Intelligencer 1, 34 (1995)).

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.

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.

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 dissociation into a mixture of atoms and ions.

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.

Applications of VB theory

An important aspect of the VB theory is the condition of maximum overlap which leads to the formation of the strongest possible bonds. This theory is used to explain the covalent bond formation in many molecules.

For Example in the case of F2 molecule the F - F bond is formed by the overlap of p orbitals of the two F atoms each containing an unpaired electron. Since the nature of the overlapping orbitals are different in H2 and F2 molecules, the bond strength and bond lengths differ between H 2 and F2 molecules.

In HF molecule the covalent bond is formed by the overlap pf 1s of H and 2p orbital of F each containing an unpaired electron. Mutual sharing of electrons between H and F results in a covalent bond between HF.

References

1. ^ Murrel, JN, Kettle, SF Tedder, JM "The Chemical Bond", John Wiley & Sons (1985) ISBN 0-471-90759-6
2. ^ Walter Heitler - Key participants in the development of Linus Pauling's The Nature of the Chemical Bond.
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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.
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molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds.[1][2] In organic chemistry and biochemistry, the term molecule
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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. Over the last century, the concept of valence evolved into a range of approaches for describing the chemical bond,
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octet rule, and is a stable molecule.]] 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, giving them the same electronic configuration as a noble gas.
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In chemistry, molecular orbital theory (MO theory) is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule.
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Gilbert Newton Lewis (October 23, 1875 - March 23, 1946) was a famous American physical chemist known for his 1902 Lewis dot structures, his 1916 paper "The Atom and the Molecule", which is the foundation of modern valence bond theory, developed in coordination with Irving
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Lewis structures, also called Lewis-dot diagrams, 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 [1] [2].
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Walter Heinrich Heitler (2 January 1904 in Karlsruhe, Germany – 15 November 1981 in Zollikon near Zürich) was a German physicist who made contributions to quantum electrodynamics and quantum field theory.
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Fritz Wolfgang London
Born March 7, 1900
Breslau, Germany
Died March 30, 1954
Durham, North Carolina
Residence USA
Citizenship German, later USA
Field Theoretical Physics
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Walter Heinrich Heitler (2 January 1904 in Karlsruhe, Germany – 15 November 1981 in Zollikon near Zürich) was a German physicist who made contributions to quantum electrodynamics and quantum field theory.
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Schrödinger equation, proposed by the Austrian physicist Erwin Schrödinger in 1926, describes the space- and time-dependence of quantum mechanical systems. It is of central importance in non-relativistic quantum mechanics, playing a role for microscopic particles analogous to
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A wave function is a mathematical tool that quantum mechanics uses to describe any physical system. It is a function from a space that consists of the possible states of the system into the complex numbers. The laws of quantum mechanics (i.e.
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Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds.
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Fritz Wolfgang London
Born March 7, 1900
Breslau, Germany
Died March 30, 1954
Durham, North Carolina
Residence USA
Citizenship German, later USA
Field Theoretical Physics
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Linus Pauling

Linus Pauling in 1954
Born January 28 1901(1901--)
Oswego, Oregon, U.S.
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Resonance in chemistry is a tool used to represent and model certain types of non-classical molecular structures.
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hybridisation or hybridization (see also spelling differences) is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties.
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Charles Alfred Coulson FRS (1910-1974) was a prominent researcher in the field of theoretical chemistry.

Educated at Cambridge University, Coulson’s interests included mathematics, physics, chemistry, and molecular biology.
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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. Over the last century, the concept of valence evolved into a range of approaches for describing the chemical bond,
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Lewis structures, also called Lewis-dot diagrams, 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 [1] [2].
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Resonance in chemistry is a tool used to represent and model certain types of non-classical molecular structures.
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An atomic orbital is a mathematical description of the region in which an electron may be found around a single atom.[1] Specifically, atomic orbitals are the possible quantum states of the individual electrons in the electron cloud around a single atom.
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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.
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Probability is the likelihood that something is the case or will happen. Probability theory is used extensively in areas such as statistics, mathematics, science and philosophy to draw conclusions about the likelihood of potential events and the underlying mechanics of
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Electron

Theoretical estimates of the electron density for the first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: First
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stationary state is an eigenstate of a Hamiltonian, or in other words, a state of definite energy. It is called stationary because the corresponding probability density has no time dependence.
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sigma bonds (σ bonds) are a type of covalent chemical bond. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is symmetrical with respect to rotation about the bond axis.
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pi bonds (π bonds) are covalent chemical bonds where two lobes of one involved electron orbital overlap two lobes of the other involved electron orbital. Only one of the orbital's nodal planes passes through both of the involved nuclei.
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hybridisation or hybridization (see also spelling differences) is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties.
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