Valence bond theory

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. 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.