Overlapping of Atomic Orbitals

Overlapping of Atomic Orbitals

When two atoms come close to each other, there is overlapping of atomic orbitals. This overlap may be positive, negative or zero depending upon the properties of overlapping of atomic orbitals. The various arrangements of s and p orbitals resulting in positive, negative and zero overlap are depicted in below figure.

The criterion of overlap, as the main factor for the formation of covalent bonds applies uniformly to the homonuclear/heteronuclear diatomic molecules and polyatomic molecules. In the case of polyatomic molecules like CH₄, NH₃ and H₂O, the VB theory has to account for their characteristic shapes as well. We know that the shapes of CH₄, NH₃ and H₂O molecules are tetrahedral, pyramidal and bent respectively. It would be therefore interesting to find out if these geometrical shapes can be explained in terms of the orbital overlaps.

Let us first consider the CH₄ (methane) molecule. The electronic configuration of carbon in its grounds state is ⦗He⦘2s² 2p² which in the excited state becomes ⦗He⦘2s¹ 2p_x¹ 2p_y¹ 2p_z¹. The energy required for this excitation is compensated by the release of energy due to overlap between the orbitals of carbon and the hydrogen. The four atomic orbitals of carbon, each with an unpaired electron can overlap with the 1s orbitals of the four H atoms which are also singly occupied. This will result in the formation of four C-H bonds. It will, however, be observed that while the three p orbitals  of carbon are at 90 to one another , the HCH angle for these will also be 90. That is three C-H bonds will be oriented at 90 to one another. The 2s orbital of carbon and the 1s orbital of H are spherically symmetrical and they can overlap in any direction. Therefore the direction of the fourth C-H bond cannot be ascertained. This description does not fit in with the tetrahedral HCH angles of 109.5. Clearly, it follows that simple atomic orbital overlap does not account for the directional characteristics of bonds in CH₄. Using similar procedure and arguments, it can be seen that in the case of NH₃ and H₂O molecules, the HNH and HOH angles should be 90. This is in disagreement with the actual bond angles of 107 and 104.5 in the NH₃ and H₂O molecules respectively.