Valence Bond Theory

Valence bond (VBT) theory is one of two main theories in chemistry, along with molecular orbital (MO) theory, that was established to describe chemical bonding using quantum mechanics approaches. When a molecule is formed, the atomic orbitals of the fragmented atoms unite to form individual chemical bonds. 

HISTORY–

The Lewis approach to the chemical bonding failed to provide insight into how chemical bonds are formed. In addition, the valence shell electron pair repulsion theory (commonly known as VSEPR theory) had just a few applications (and also failed in predicting the geometry corresponding to complex molecules).

The valence bond hypothesis was proposed by German physicists Walter Heinrich Heitler and Fritz Wolfgang London to address these concerns. The development of the covalent bond between the two hydrogen atoms was also explained using the Schrodinger wave equation. The valence bond hypothesis is used to depict the chemical bonding of two hydrogen atoms.

The ideas of the electronic configuration, atomic orbitals (and their overlapping), and atomic orbital hybridization are all included in this theory. 

The electrical structure of molecules created by this overlapping of atomic orbitals is also explained by the valence bond theory. It also emphasizes how one atom’s nucleus is attracted to the electrons of the other atoms in a molecule.

Theory–

The overlap of half-filled valence atomic orbitals of each atom having one unpaired electron forms a covalent connection between two atoms, according to this theory. A valence bond structure is similar to a Lewis structure, however it is employed in situations where a single Lewis structure cannot be stated. Each of these VB structures constitutes a Lewis structure in its own right. The main point of resonance theory is the combination of valence bond configurations. The overlapping atomic orbitals of the interacting atoms form a chemical connection, according to the Valence Bond Theory.

Because of the overlapping, electrons are most likely to be in the bond region. Bonds are viewed as weakly connected orbitals in the Valence Bond Theory (small overlap). In-ground state molecules, valence bond theory is usually easier to apply. During bond formation, the core orbitals and electrons are basically unchanged.

Postulates of valence bond theory-

• When two valence orbitals (half-filled) from two separate atoms overlap on one other, covalent bonds are created. As a result of this overlapping occurring, the electron density in the area between the two bonding atoms increases, boosting the stability of the resulting molecule.

• The atom’s valence shell has several unpaired electrons, allowing it to make many bonds with other atoms. According to the valence bond theory, the paired electrons present within the valence shell do not participate in the creation of chemical bonds.

• Chemical bonds that are covalent in nature are directed and parallel to the region corresponding to the overlapping atomic orbitals.

• The sigma bonds and the pi bonds are distinguished by the pattern in which the atomic orbitals overlap, i.e. pi bonds are produced by overlapping along the axis containing the nuclei of the two atoms, whereas sigma bonds are formed by overlapping along the axis containing the nuclei of the two atoms. 

Below is a diagram of how sigma and pi bonds are formed.

The atomic orbital overlap on the bond formation.

The atomic orbitals that overlap can be different. Sigma and pi are the two forms of overlapping orbitals. When the orbitals of two shared electrons cross in front of each other, a sigma bond is formed. When two parallel orbitals intersect, they form a pi bond. Because two spheres are always coaxial, a bond between two s-orbital electrons is a sigma bond. Single bonds have one sigma bond, double bonds have one sigma bond and one pi bond, and triple bonds have one sigma bond and two pi bonds, in that sequence. The atomic orbitals for bonding, on the other hand, could be hybrids. The bonding atomic orbitals frequently take on the characteristics of multiple different types of orbitals. Hybridization is a technique for obtaining an atomic orbital with the correct character for bonding.

The larger the overlap, the stronger the bond.

In general, the stronger the link created between the two atoms is the higher the overlap. As a result of the orbital overlap idea, atoms interact by overlapping their orbitals, generating a lower energy state in which their valence electrons with opposing spin team up to create a covalent bond.

CONCLUSION–

The overlap of two independent atomic orbitals on different atoms provides an area with one pair of electrons shared by the two atoms, according to the Valence Bond Theory. A bond is formed when the orbitals overlap along an axis that contains the nuclei. They establish a bond when they overlap in a way that generates a node along this axis.

The criterion of maximal overlap, which leads to the development of the strongest possible bonds, is a fundamental feature of the valence bond theory. This idea is used to explain the development of covalent bonds in a variety of compounds.