In chemistry, valence bond theory explains the nature of a chemical bond in a molecule in terms of atomic valencies. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties A chemical bond is the physical process responsible for the attractive interactions between Atoms and Molecules and which confers stability to diatomic and polyatomic In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by In Chemistry, valence, also known as valency or valency number, is a measure of the number of Chemical bonds formed by the Atoms  Valence bond theory summarizes the rule that the central atom in a molecule tends to form electron pair bonds in accordance with geometric constraints as defined by the octet rule, approximately. 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 Valence bond theory is closely related to molecular orbital theory. In Chemistry, molecular orbital theory ( MO theory) is a method for determining molecular structure in which Electrons are not assigned to individual
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. Gilbert Newton Lewis ( October 23, 1875 - March 23, 1946) was a famous American physical chemist known for the discovery Lewis structures, also called Lewis-dot diagrams are diagrams that show the bonding between Atoms of a 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. Walter Heinrich Heitler ( 2 January 1904 &ndash 15 November 1981) was a German physicist who made contributions to Quantum electrodynamics Fritz Wolfgang London ( March 7, 1900 &ndash March 30, 1954) was a German -born American theoretical Physicist. 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. Walter Heinrich Heitler ( 2 January 1904 &ndash 15 November 1981) was a German physicist who made contributions to Quantum electrodynamics In Physics, especially Quantum mechanics, the Schrödinger equation is an equation that describes how the Quantum state of a Physical system A wave function or wavefunction is a mathematical tool used in Quantum mechanics to describe any physical system He then called up his associate Fritz London and they worked out the details of the theory over the course of the night. Fritz Wolfgang London ( March 7, 1900 &ndash March 30, 1954) was a German -born American theoretical Physicist.  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). Linus Carl Pauling (February 28 1901 – August 19 1994 was an American Scientist, Peace activist, Author and educator. Resonance in Chemistry is a theory used to represent and model certain types of non-classical Molecular structures Resonance is a key component -->In Chemistry According to Charles Coulson, author of the noted 1952 book Valence, this period marks the start of “modern valence bond theory”, as contrasted with older valence bond theories, which are essentially electronic theories of valence couched in pre-wave-mechanical terms. Charles Alfred Coulson FRS ( 13 December 1910 - 7 January 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 Resonance theory was criticized as imperfect by Soviet chemists during the 1950's. 
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. Lewis structures, also called Lewis-dot diagrams are diagrams that show the bonding between Atoms of a Each of these VB structures represents a specific Lewis structure. This combination of valence bond structures is the main point of resonance theory. Resonance in Chemistry is a theory used to represent and model certain types of non-classical Molecular structures Resonance is a key component Valence bond theory considers that the overlapping atomic orbitals of the participating atoms form a chemical bond. An atomic orbital is a Mathematical function that describes the wave-like behavior of an electron in an atom A chemical bond is the physical process responsible for the attractive interactions between Atoms and Molecules and which confers stability to diatomic and polyatomic Because of the overlapping, it is most probable that electrons should be in the bond region. Probability is the likelihood or chance that something is the case or will happen The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Valence bond theory views bonds as weakly coupled orbitals (small overlap). Valence bond theory is typically easier to employ in ground state molecules. In Quantum mechanics, a stationary state is an Eigenstate of a Hamiltonian, or in other words a state of definite energy
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. In Chemistry, sigma bonds ( σ bonds) are the strongest type of covalent Chemical bond. Pi bonds occur when two orbitals overlap when they are parallel. In Chemistry, pi bonds ( π bonds) are covalent Chemical bonds where two lobes of one involved electron orbital overlap two lobes 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. -->In Chemistry
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. In Chemistry, a molecular orbital (or MO) is a region in which an Electron may be found in a Molecule. In Chemistry, a molecular orbital (or MO) is a region in which an Electron may be found in a Molecule. MO theory can predict magnetic properties in a straight forward manner, but valence bond theory is more complicated although giving the similar results. In Physics, magnetism is one of the Phenomena by which Materials exert attractive or repulsive Forces on other Materials. 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. Mathematics is the body of Knowledge and Academic discipline that studies such concepts as Quantity, Structure, Space and 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. Modern valence bond theory is the term used to describe applications of Valence bond theory with computer programs that are competitive in accuracy and economy with programs This replaces the overlapping atomic orbitals by overlapping valence bond orbitals that are expanded over all basis functions in the molecule. A basis set in Chemistry is a set of functions used to create the Molecular orbitals which are expanded as a linear combination of such functions with the The resulting energies are more competitive with energies where electron correlation is introduced based on a Hartree-Fock reference wavefunction. 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) method is an approximate method for the determination of the ground-state
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 a HF molecule the covalent bond is formed by the overlap of 1s orbital 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.