In order to explain the characteristic geometrical shapes of polyatomic molecules like CH4, NH3 and H2O etc., Pauling introduced the concept of hybridization. According to him the atomic orbitals combine to form new set of equivalent orbitals known as hybrid orbitals. Unlike pure orbitals, the hybrid orbitals are used in bond formation. The phenomenon is knows as hybridization which can be defined as the process of intermixing of the orbitals of slightly different energies so as to redistribute their energies, resulting in the formation of new set of orbitals of equivalent energies and shape. For example when one 2s and three 2p-orbitals of carbon hybridise, there is the formation of four new sp3 hybrid orbitals.

Salient features of hybridization:

The main features of hybridization are as under:
  1. The number of hybrid orbitals is equal to the number of the atomic orbitals that get hybridized.
  2. The hybridized orbitals are always equivalent in energy and shape.
  3. The hybrid orbitals are more effective in forming stable bonds than the pure atomic orbitals.
  4. These hybrid orbitals are directed in space in some preferred direction to have minimum repulsion between electron pairs and thus a stable arrangement. Therefore, the type of hybridization indicates the geometry of the molecules.

Important conditions for hybridization

  • The orbitals present in the valence shell of the atoms are hybridized.
  • The orbitals undergoing hybridization should have almost equal energy.
  • Promotion of electron is not essential condition prior to hybridization.
  • It is not necessary that only half filled orbitals participate in hybridization. In some cases, even filled orbitals of valence shell take part in hybridization.

Types of Hybridization

There are various types of hybridization involving s, p, and d orbitals. The different types of hybridization are as under.

sp hybridization: This type of hybridization involves the mixing of one s and one p orbital resulting  in the formation of two equivalent sp hybrid orbitals. The suitable orbitals for sp hybridization are s and pz, if the hybrid orbitals are to lie along the z-axis. Each sp hybrid orbitals has 50% s-character and 50% p-character. Such a molecule in which the central atom is sp-hybridized and linked directly to two other central atoms possesses liner geometry. This type of hybridization is also known as diagonal hybridization.

The two sp hybrids point in the opposite direction along the z-axis with projecting positive lobes and very small negative lobes, which provides more effective overlapping resulting in the formation of stronger bonds.

Example of molecule having sp hybridization
BeCl2: The ground state electronic configuration of Be is 1s22s2. In the exited state one of the 2s-electrons is promoted to vacant 2p orbital to account for its divalency. One 2s and one 2p-orbitals get hybridized to form two sp hybridized orbitals. These two sp hybrid orbitals are oriented in opposite direction forming an angle of 180. Each of the sp hybridized orbital overlaps with the 2p-orbital of chlorine axially and form two Be-Cl sigma bonds. This is shown below figure.
sp hybridization,  hybridization

Sp2 hybridization : In this hybridization there is involvement of one s and two p-orbitals in order to form three equivalent sp2 hybridized orbitals. For example, in BCl3 molecule, the ground state electronic configuration of central boron atom is 1s22s22p1. In the excited state, one of the 2s electrons is promoted to vacant 2p orbitals as a result boron has three unpaired electrons. These three orbitals (one 2s and two 2p) hybridise to form three sp2 hybrid orbitals. The three hybrid orbitals so formed are oriented in a trigonal planar arrangement and overlap with 2p orbitals of chlorine to form three B-Cl bonds. Therefore, in BCl3 the geometry is trigonal planar with CIBCl bond of 120.
Sp2 hybridization

Sp3 hybridizationThis type of hybridization can be explained by taking the example of CH4 molecule in which three is mixing of one s-orbital and three p-orbitals of the valence shell to form four sp3 hybrid orbital of equivalent energies and shape. There is 25% s-character and 75% p-character in each sp3 hybrid orbital. The four sp3 hybrid orbitals so formed are directed towards the four corners of the tetrahedron. The angle between sp3 hybrid orbital is 109.5 as shown below.
Sp3 hybridization

The structure of NH3 and H2O molecules can also be explained with the help of sp3 hybridization.  In NH3, the valence shell (outer) electronic configuration of nitrogen in the grounds state is 2s2  2p1x  2p1y  2p1z having three unpaired electrons in the sp3 hybrid orbitals and a lone pair of electrons is present in the fourth one. These three hybrid orbitals overlap with 1s orbitals of hydrogen atoms to form three N-H sigma bonds.  As we know that the force of repulsion between a lone pair and a bond pair is more than the force of repulsion between two bond pairs of electrons. The molecule thus gets distorted and the bond angle is reduced to 107 from 109.5. The geometry of such a molecule will be pyramidal as shown below.
The structure of NH3 and H2O molecules

In the case of H2O molecule, the four oxygen orbitals (one 2s and three 2p) undergo sp3 hybridization forming four sp3 hybrid orbitals out of which two contain one electron each and the other two contain a pair of electrons. These four sp3 hybrid orbitals acquire a tetrahedral geometry, with two corners occupied by hydrogen atoms while the other two by the lone pairs. The bond angle in this case is reduced to 104.5 from 109.5 and the molecule thus acquires a V-shape or angular geometry.

H2O molecule