Hybridization

The structures of different molecules can be explained on the basis of hybridization. For e.g., in case of carbon, the ground state electronic

To explain the tetravalency of carbon, it was proposed that one of the electrons from 2s filled orbital is promoted to the 2p empty orbital (2p_z), which is in a higher energy state. Thus, four half-filled orbitals form in the valence shell this accounts for the bonding capacity of four carbon atoms. This state is known as excited state and the configuration of carbon in the excited state is:

The above configuration reveals that all the four bonds formed by carbon will not be identical. For e.g., in the formation of CH₄ molecule, one C-H bond will be formed by the overlapping of 2s-orbital of C and 1s-orbital of H whereas the other three C-H bonds will be formed by the overlapping of 2p-orbitals of C and 1s-orbital of H. Therefore, all the bonds will not be equivalent.

But actually, in most of the carbon compounds, such as methane (CH₄), carbon tetrachloride (CCl₄) etc., all the four bonds of carbon atom are equivalent. The equivalent character of the bonds can be explained with the help of hybridisation.

Hybridisation may be defined as the phenomenon of intermixing of the orbitals of slightly different energies so as to redistribute their energies and to give new set of orbitals of equivalent energy and shape. The new orbitals formed as a result of hybridization are called hybrid or hybridized orbitals. Thus, to form four equivalent bonds, one 2s and three 2p-orbitals of carbon hybridize and form four new orbitals.such orbitals are called sp³ hybrid orbitals.

The important characteristics of hybridisation are listed below:

(i) The number of hybridized orbitals formed is equal to the number of orbitals that get hybridized.

(ii) The hybridized orbitals are always equivalent in energy and shape.

(iii) The hybrid orbitals are more effective in forming stable bonds than the pure atomic orbitals.

(iv) The hybrid orbitals are directed in space in some preferred directions to have stable arrangements.

Therefore, the type of hybridization gives the geometry of the molecule. Depending upon the different combinations of s- and three p-orbitals, three types of hybridizations are known.

sp hybridization

This involves the mixing of one s- and one p-orbital forming two sp-hybrid orbitals. The two sp-hybrid orbitals are oriented in a linear arrangement and bond angle is 180°. For e.g., BeF₂ involves sp-hybridization and is, therefore, linear.

sp² hybridization

In this case, one s- and two p-orbitals hybridize to form three sp² hybrid orbitals. These three sp² hybrid orbitals are oriented in a trigonal planar arrangement. For e.g., in BH₃ boron atom undergoes sp² hybridization and therefore, BH₃ has trigonal planar geometry and HBH bond angle is 120^o.

sp³ hybridization

In this case, one s- and three p-orbitals hybridize to form four sp³ hybrid orbitals. These four sp³-hybrid orbitals are oriented in a tetrahedral arrangement. The common example of molecule involving sp³-hybridisation is methane (CH₄). Therefore, CH₄ has tetrahedral geometry and HCH bond angle is 109.5^o.

Hybrid orbitals and molecular shapes involving s and p-orbitals