Chemical bonding is explained using the valence bond and molecular orbital theories. A chemical bond can be formed when the orbitals of two atoms with unpaired electrons overlap. When two atomic orbitals intersect between the nuclei of two atoms, a sigma bond () is formed. When two atomic orbitals intersect outside of the space between the nuclei, pi bonds form (outside of the internuclear axis).
The strongest bonds are formed when the orbitals overlap the most. Valence bond (VB) theory, together with molecular orbital (MO) theory, is one of two basic theories in chemistry that uses quantum mechanics to describe chemical bonding. A covalent bond is formed by the physical clash of half-filled valence orbitals in two or more atoms, as per VB theory.
The crossover of half-filled atomic orbitals (each holding a single electron) yields a pair of electrons amongst both linked atoms, according to the Valence Bond Theory. When a part of one orbital and a section of a second orbital occupies the same region of space on two separate atoms, we say they overlap. A covalent bond is formed when two criteria are met: (1) an orbital on one atom intersects an orbital on another atom, and (2) the solitary electrons in each orbital unite to create an electron pair, according to valence bond theory.
The mutual interest here between the negatively charged electron pair as well as the positively charged nuclei of the two atoms creates a covalent bond that physically connects the two atoms. The level of overlap of the orbitals engaged determines the strength of a covalent bond. Orbitals with a lot of overlap form stronger bonds than orbitals with less overlap.
The valence bond theory’s main postulates are mentioned below.
The head-to-head collision of the atomic orbitals involved in the bond forms sigma bonds. Pi bonds, but at the other end, involve the overlapping of atomic orbitals in a parallel manner.
Modern valence bond theory now works in tandem with molecular orbital theory, which rejects the valence bond assumption that electron pairs are isolated between two specific atoms in a molecule and instead believes that they are distributed in sets of molecular orbitals that can span the entire molecule. The molecular orbital theory provides a simple way to predict magnetic and ionization properties, whereas valence bond theory provides similar conclusions but is more difficult. Aromatic characteristics of molecules are attributed to spin coupling of orbitals in modern valence bond theory.
The valence bond theory depicts the reorganization of electrical charge that occurs when bonds are broken and established during the course of a chemical process considerably more accurately. Valence bond theory, in an instance, correctly predicts the breakdown of homonuclear diatomic molecules into individual atoms, whereas basic molecular orbital theory predicts a mixture of atoms and ions. For example, because dihydrogen’s molecular orbital function contains an equal blend of covalent and ionic valence bond structures, it mistakenly predicts that the molecule will disintegrate into an equal mixture of hydrogen atoms and hydrogen positively and negatively ions.
Consider a more complicated molecule, such as water. The valence-shell electron structure of each hydrogen atom is 1s1. The electron configuration of oxygen’s valence shell is 2s2, 2px2, 2py1, 2pz1. So, as long as the electrons in the oxygen and hydrogen possess opposite spins, we have two unoccupied electron locations in the oxygen that can possibly form bonds with two hydrogen atoms.
It demonstrates how the spins of the hydrogen 1s atomic orbitals must be opposing in order for them to overlap with oxygen 2p atomic orbitals and form bonds. The optimal distance/overlap among the atoms here for a minimum in PE for an O-H bond is 96 pm, which is different from the optimal distance/overlap between the atoms for an H-H bond.
Chemical bonding is described by this hypothesis. According to VBT, the creation of a chemical bond between two atoms is caused by the overlap of imprecisely filled atomic orbitals. The lone electrons are shared, resulting in the generation of a hybrid orbital.
The valence bond theory fails to explain carbon's tetravalency and provides no insight into the energies associated with electrons. The theory implies that electrons are concentrated in specific regions.
The VBT’s maximum overlap condition can be used to elucidate how covalent bonds are generated in a variety of compounds. The theory might provide information.
Disadvantage
Valence Bond’s idea is not without flaws. It comes with its own set of constraints. They are as follows:
It is unable to explain carbon’s tetravalency.
The energy of electrons is not discussed in this theory.
The electrons are thought to be localized to specific regions, according to the assumptions.