Valence bond theory
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The valence bond theory considers the overlap of the atomic orbitals of the participation atoms to form a chemical bond. Due to the overlapping, it is most probable that electrons should be in the bond region.
The overlapping atomic orbitals can be of different types. There are two different types of overlapping orbitals: sigma and pi. Sigma bonds occur when two orbitals overlap (pair of shared electrons) occur on the axis. Pi bonds occur when two orbitals overlap but do not do so on the axes. For example, what is said to be a sigma bond is the frontal overlapping of atomic orbitals.
However, the atomic orbitals for bonding may not be "pure" atomic orbitals directly from the solution of the Schrödinger equation. 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 hybridisation.
Valence bond theory has been extended to Molecular Orbital Theory, which explains this hybridisation as linear combinations of the wavefunctions associated with each atom involved.