Orbital overlap concept
If we refer to the minimum energy state in the formation of hydrogen molecule the two H-atoms are enough near so as to allow their atomic orbital to undergo partial interpenetration. Partial interpenetration means that a part of electron cloud of each of the two half filled atomic orbitals become common. This partial interpenetration of atomic orbitals is called overlapping of atomic orbitals. The electrons belonging to these orbitals are said to be shared. The probability of finding the shared electrons is maximum in the region of overlap than at other places. This lead us to the conclusion that orbital overlap is necessary for the electrons to be shared or for the bond to be formed.
According to Pauli’s principle the two electrons occupying same region of space must have different quantum numbers. Therefore the two electrons forming a bond must have opposite spins. The overlapping at atomic orbitals in case of hydrogen molecule has already been discussed.
The criterion of overlap of atomic orbitals of combining atoms is the main factor for the formation of covalent bonds both in homo nuclear as well as in hetero nuclear diatomic molecules and polyatomic molecules. In case of polyatomic molecules like CH4, NH3 and H2O, the VBT should be taken into consideration for their characteristic shapes. As we know the shapes of CH4, NH3 and H2O molecules are tetrahedral , pyramidal and bent respectively. Their shapes can be described in terms of the orbital overlaps.
In CH4, the electronic configuration of carbon in its ground state is [He]2s2 2p2 which is excited state becomes [He] 2s1 
. The four atomic orbitals of carbon, each with one unpaired electron can overlap with 1s-orbitals of the four H-atoms which are also singly occupied; consequently, four C-H bonds can be formed. The three p-orbitals of carbon are at 900 to one another, the HCH angle for these will also be 900. But 2s-orbital of carbon and 1s-orbtial of H, both the spherical and can overlap in any direction. Therefore, direction of the fourth C-H bond cannot be determined. This explanation does not fit in with the tetrahedral HCH angles of 109.50. So we can say that simple atomic orbital overlap does not account for the directional characteristics of bonds in CH4. Similarly in case of NH3 and H2O molecules, the HNH and H-O-H bond angles should be 900, which does not agree with the actual bond angles of 1070 and 104.50 in NH3 and H2O molecules respectively.