Valence bond theory is a chemicalbonding theory that explains the bondingbetween two atoms is caused by the overlap of half-filled atomic orbitals. The two atoms share each other's unpaired electron to form a filled orbital to form a hybrid orbital and bondtogether.

Valency bond theory :
valency bond theory is a chemical bond theory that explains the bonding between two atoms is caused by the overlap of half-filled atomic orbital .The two atoms share each other unpaired electron to from a filled orbital to from a hybrid orbital and bond together.

postulates of valency bond theory :
1) The overlapping of two half-filled valence orbital of two different atom results in the formation of the covalent bond .The overlapping causes the electron density between two bonded to increase .This gives the property of stability to the molecule.
2) In case the atomic orbital posess more than one unpaired electron ,more than one bond can be from an electron paired in the valency shell cannot take part in such a bond formation.
3) The covalent bond is directional. such a bond is also parallel to be region of overlapping atomic orbital.

Limitations of valency bond theory :
1) It fails to explain the tetravalency of carbon.
2) This theory does not discuss the electron energies.
3) The assumption are about the electrons being the localized to specific location.