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In chemistry, valence bond (VB) theory is one of two basic theories—along with molecular orbital (MO) theory—that use quantum mechanics to explain chemical bonding. According to VB theory, a covalent bond forms from the physical overlap of half-filled valence orbitals in two atoms.
Mechanism of Bonding in VB Theory
The VB theory describes the formation of covalent bonds from the overlap of atomic orbitals on two different atoms. Because of the overlap, it is highly probable that a pair of electrons are found in the physical region or space where the orbitals overlap.
Sigma ($\sigma$) and Pi ($\pi$) Bonds
There are two types of overlapping orbitals: sigma ($\sigma$) and pi ($\pi$). Both bonds are formed from the overlap of two orbitals, one on each atom. $\sigma$ bonds occur when orbitals overlap between the nuclei of two atoms, also known as the internuclear axis.
$\pi$ bonds occur when two (unhybridized) p-orbitals overlap. The p-orbitals, in one $\pi$ bond, are located above and below the nuclei of the atoms. By occupying the region of space that is above, below, and on the sides of an atom's nuclei, two $\pi$ bonds can form.
VB theory complements molecular orbital (MO) theory, which does not adhere to the VB concept that electron pairs are localized between two specific atoms in a molecule. MO theory states that electrons are distributed in sets of molecular orbitals that can extend over the entire molecule. MO theory can predict magnetic and ionization properties in a straightforward manner. VB theory produces similar results, but is more complicated.
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. In the F2 molecule, the F–F $\sigma$ covalent bond is formed by the overlap of pz orbitals of the two F atoms, each containing an unpaired electron. Since the nature of the overlapping orbitals is different in H2 and F2 molecules, bond strength and bond lengths differ between H2 and F2 molecules.
In an HF molecule, the covalent $\sigma$ bond forms from the overlap of the 1s orbital of H and the 2pz orbital of F, each containing an unpaired electron. Mutual sharing of electrons between H and F results in a covalent bond in HF.
Source: Boundless. “Explanation of Valence Bond Theory.” Boundless Chemistry. Boundless, 19 Aug. 2016. Retrieved 26 Sep. 2016 from https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/advanced-concepts-of-chemical-bonding-10/valence-bond-theory-81/explanation-of-valence-bond-theory-360-3680/