ΓΌ  Valence Bond Theory (VBT): This theory was introduced by Heitler and London and later developed by Linus Pauling in order to explain the shape of molecules theoretically. This theory can be described in following points.
(i) A covalent bond is formed when the orbital of one atom is situated in such a way that it overlaps with the orbital of another atom, each of them containing one unpaired electron.
(ii) The atomic orbitals overlap and the overlapped region is occupied by both the electrons. These two electrons must have opposite spin.
(iii) As a result of this overlapping, there is maximum electron density between the two atoms. A large part of the binding force of covalent bond results from the attraction of these electrons by the nuclei of both the atoms.
(iv) Atoms maintain their individuality. When the bond is formed, only valence electrons from each bonded atom are involved and the inner atomic orbitals of each atom remain undisturbed.  
Consider 2 hydrogen atoms A and B with nucleus NA and NB and electrons eA and eB respectively. When the two atoms are at a large distance from each other, there is no further interaction between them. Due to this their potential energy is zero. When the two atoms start approaching each other, new attractive and repulsive forces begin to operate.
The attractive forces arise between the nucleus of one atom and electron of another atom i.e., NA– eB and NB– eA. Similarly, repulsive forces arise between electrons of two atoms i.e., eA – eB, and the nuclei of two atoms NA – NB. Attractive forces bring the two atoms close to each other whereas repulsive forces push them away.
The magnitude of new attractive force is more than the new repulsive forces. So, the two atoms approach each other and potential energy decreases. At a particular stage, the net attractive force balances the net repulsive forces and the energy becomes minimum. At this stage, the hydrogen atoms are said to be bonded together to form a stable molecule.



Let us discuss another example of formation of He2 molecule in which number of attractive forces are less in comparison to number of repulsive interaction so the He2 molecule does not exists.