Valence Bond Theory

Valence bond theory explains the formation of covalent bond.  According to Heitler and London the covalent Bond forms by overlapping of electron cloud of the atoms. Only the condition of forming covalent bond between two atoms is the overlapping of electron clouds of two atom with each others. Methane is a compound with carbon and hydrogen. The bond between carbon and hydrogen is covalent bond. This bonds forms by overlapping of electron cloud of carbon with the electron cloud of hydrogen. How strong will be the covalent bond? It depends on the nature of overlapping. Maximum overlapping of electron cloud ensures maximum strength of bond. The principle discussion of valence bond theory is the overlapping of electron clouds. Overlapping also follows some conditions. These are as follows. The orbital that will participate in overlapping to form bond must have only one electron. Orbital containing a pair of electron can not do this. To make an effective overlapping atoms with valence electron must have to sufficiently closer to each other. Though Valence Bond Theory explains the causes of covalent bond, but yet it have some limitation. The limitation of valence bond theory is explained here.Presence of other nuclei changed the electronic arrangement of molecules.  Valence bond theory can not explain the paramagnetic character of oxygen. In the study of chemistry there may found some electron deficient compound in nature. Valence Bond Theory can not explains the bonding of these compounds. Valence Bond Theory can not explains the all properties of the compounds with resonating structure.