Hybridisation | Physical Chemistry for NEET PDF Download

What is Hybridisation?

Hybridisation is defined as the mixing of the atomic orbitals belonging to the same atom but having slightly different energies so that redistribution of energy takes place between them resulting in the formation of new orbitals of equal energies and identical shapes. The new orbitals thus formed are known as hybrid orbitals.

Some Important Points about Hybridisation

• Only those orbitals which have approximately equal energies and belong to the same atom or ion can undergo hybridisation.
• The number of hybrid orbitals produced is equal to the number of atomic orbitals mixed.
• It is not necessary that all the half-filled orbitals must participate in hybridisation. Similarly, it is not necessary that only half-filled orbitals should participate in hybridisation. Even completely filled orbitals with slightly different energies can also participate.
• Hybridisation never takes place in isolated atoms but it occurs only at the time of bond formation.
• Type of hybridisation indicates the geometry of molecules. One can tell the shape of a molecule by knowing the kind of hybridisation involved.
• The bigger lobe of the hybrid orbital always has +ve sign while the smaller lobe on the opposite side has a -ve sign.

Type of Hybridisation

1. Diagonal or sp hybridisation

• When one s and one p orbital belonging to the same main shell of an atom mix together to form two new equivalent orbitals, this type of hybridisation is called sp hybridisation or diagonal hybridisation. 
• The new orbitals formed are called sp hybrid orbitals.
  

2. Trigonal or sp² hybridisation

• When one s and two p orbitals of the same shell of an atom mix to form three new equivalent orbitals, this type of hybridisation is called sp² hybridisation or trigonal hybridisation. 
• The new orbitals formed are called sp² hybrid orbitals.

In such hybridisation one s- and three p-orbitals are mixed to form four sp³_– hybrid orbitals having a tetrahedral structure with bond angle 109 degrees 28′, that is, 109.5 degrees.

Studying the Formation of Various Molecules

1) Methane

4 equivalent C-H σ bonds can be made by the interactions of C-sp3 with an H-1s

2) Ethane

6 C-H sigma(σ) bonds are made by the interaction of C-sp³ with H-1s orbitals and 1 C-C σ bond is made by the interaction of C-sp³ with another C-sp³ orbital.

3) Formation of NH₃ and  H₂O molecules

In NH₂ molecule nitrogen atom is sp³-hybridised and one hybrid orbital contains two electrons. Now three 1s- orbitals of three hydrogen atoms overlap with three sp³ hybrid orbitals to form NH₃ molecule. The angle between H-N-H should be 109.5⁰ but due to the presence of one occupied sp3-hybrid orbital the angle decreases to 107.80. Hence, the bond angle in NH3 molecule is 107.80.

4) Formation of C2H4  and C2H2 Molecules

In C2H4 molecule carbon atoms are sp2-hybridised and one 2p-orbital remains out to hybridisation. This forms p-bond while sp² –hybrid orbitals form sigma- bonds.

5) Formation of NH₃  and H₂O  Molecules by sp² hybridization

In H₂O molecule, the oxygen atom is sp³ – hybridized and has two occupied orbitals. Thus, the bond angle in the water molecule is 105.5⁰.

Predicting Hybridisation

Calculate the number of hybrid orbitals (X) to be formed by the central atom as follows:

i.e. X = 1/2 [ VE + MA - c + a]
Note that only monovalent atoms (MA) or groups are to be considered. For divalent ions, MA = 0.

• If X = 2, it means two hybrid orbitals are to be formed. Hence, hybridization is sp.
• If X = 3, it means three hybrid orbitals are to be formed. Hence, hybridisation is sp² 
• And so on, as given in the following table: