Hybridisation

Theory

To explain the characteristic geometrical shapes of polyatomic molecules, hybridisation was introduced. The atomic orbitals combine to form a new set of equivalent orbitals known as hybrid orbitals or hybridised orbitals.

Hybridisation is the process of intermixing of the orbitals of slightly different energies so as to redistributed their energies, resulting in the formation of new set of orbitals of equivalent energies and shapes.

For example, carbon atoms have one 2s and three 2p orbitals. They intermix during bonding to form four equivalent sp³ hybridised orbitals. For methane (CH₄), these orbitals overlap axially with 1s orbitals of four H atoms to form four equivalent C-H bonds.

Features of hybridisation:

• The number of hybrid orbitals is equal to the number of atomic orbitals that get hybridised.
• The hybridised orbitals are always equivalent in energy and shape.
• The hybrid orbitals are more effective in forming stable bonds than the pure atomic orbitals.
• The hybrid orbitals are directed in space in some preferred directionto have minimum repulsion between electron pairs.

Types of hybridisation:

1. sp-hybridisation: 

• Simplest type of hybridisation involving one s and one p-orbitals.
• The two sp-hybrid orbitals are oriented in a straight line making an angle of 180^o.
• The molecule possesses linear geometry.
• 50 % s-character and 50 % p-character.

Example of molecules having sp-hybridisation are BeF₂, BeCl₂, BeH₂, HCCH, CO₂, etc

2. sp²-hybridisation: 

• Hybridisation involves one s and two p (p_x and p_y) orbitals.
• The three sp²-hybrid orbitals are directed towards the three corners of an equilateral triangle making an angle of 120^o.
• The molecule possesses triangular geometry.
• 33.33 % s-character and 66.67 % p-character.

Example of molecules having sp²-hybridisation are BF₃, BCl₃, BH₃, H₂C=CH₂, SO₂, etc.

3. sp³-hybridisation: 

• Hybridisation involves one s and three p-orbitals.
• The four sp³-hybrid orbitals are directed towards the four corners of a tetrahedron making an angle of 109^o.
• The molecule possesses tetrahedron geometry.
• 25 % s-character and 75 % p-character.

Example of molecules having sp³-hybridisation are CH₄, CCl₄, SiCl₄, NH₃, H₃C-CH₃, H₂O, etc.

Hybridisation in NH₃:

Hybridisation in H₂O:

Hybridisation involving d-orbitals:

When d-orbitals are lying vacant or half-filled, then d-orbitals can participate in hybridisation.

The energy of 3d orbitals is comparable to 3s and 3p orbitals as well as 4s and 4p orbitals. So, 3d orbitals can be involved in hybridisation with either of these orbitals.

We will only concentrate on sp³d and sp³d² hybridisation where hybridisation involves outer d-orbitals.

sp³d-Hybridisation:

• Involves mixing of one s, three p and one d-orbitals.
• Trigonal bipyramidal geometry.
• Equatorial orbitals lie in horizontal plane with 120^o angle with one another.
• Axial orbitals lie in vertical plane at right angle to the plane of equatorial orbitals.

Formation of PCl₅:

sp³d²-Hybridisation:

• Involves mixing of one s, three p and two d-orbitals.
• Orbitals are directed towards the corners of octahedron.
• Octahedron geometry.
• Orbitals lie at 90^o angle to one another.