In day to day life, we are witnessed with a variety of colours around us. Don’t you find this interesting? Have you even wonder how we are able to see so many colours? This all happens because of hybridisation of universal colours to get variety of colours which makes our surrounding look more beautiful and colourful. 

Let’s understand this with an example:
Start with one beaker of 50 mL red water that represents the 2s orbital and three beakers, each with 50 mL blue water, that represent three 2p orbitals. Mix all four beakers then we will get 200 mL of purple water.




Why do we study hybridisation?

In methane, there should be one s-s overlapping and three -p  overlapping, for formation of four C - H bonds. it is assumed that the energy, and size of these orbitals should not be same but experimentally we found that all the bond  parameters of C - H bond in CH₄ are same. why is this so? To figure this out, scientists had introduced a hypothetical concept which is known as hybridisation.

Table of content: 

• What is hybridisation?
• Salient features of hybridization
• Types of hybridisation
• Hybridisation involving d-orbitals
• Practice problems
• FAQs

What is hybridisation?

Hybridisation is defined as the concept of intermixing of atomic orbitals of similar or comparable energies of an atom to form a new set of almost same hybrid orbitals. The atomic orbitals of a similar energy level can only take part in hybridization and both fully filled and half-filled orbitals can also take part in this hypothetical process, provided they have comparable energy.
Example: Formation of SP² hybridisation

 
                       Hybridisation = SP²


Example: Possible mixing of orbitals

Salient features of hybridization

• Hybrid orbitals have the same shape and energy.
• Bonds formed by the hybrid orbitals are always more stable than bonds formed by the pure atomic orbital.
• The type of hybridization indicates the geometry of molecules.
• The number of hybrid orbitals is always equal to the number of atomic orbitals undergoing mixing. Also, hybrid orbitals never form pi bonds.

Note: Orientation of hybridized orbitals should be in a way to ensure minimum repulsion between electron pairs to obtain a stable geometry.

Types of Hybridisation

Naming of Hybrid Orbitals: Naming can be done on the basis of the participating atomic orbitals in hybridisation.

Based on the types of orbitals involved in mixing, hybridization can be classified as

• SP hybridisation : The intermixing of one S and one P-orbital of the comparable energy of the same shell of an atom to form two new equivalent orbitals is known as sp hybridisation. The new orbitals formed are known as sp hybridized orbitals and these forms linear geometry with an angle of 180°.
  
  
  
   
• sp² hybridisation: The intermixing of one s and two p-orbitals of comparable energy of the same shell of an atom to form three equivalent orbitals is known as sp² hybridisation. The new orbitals formed are known as sp² hybrid orbitals. The geometry formed by this sp²  orbitals is trigonal planar and is maintained at 120°.

• sp³ hybridisation: The intermixing of one s-orbital and three p-orbitals of comparable energy  belonging to the the same shell of an atom together to form four new equivalent orbitals is known as sp³ hybridization. The new orbitals formed are known as sp³ hybrid orbitals having geometry as tetrahedral.
  
  
  
  

Methods for finding hybridisation

• Orbital box diagram method: An orbital diagram, also known as an orbital box diagram, is a representation of an atom's electronic arrangement. The electrons that fill the orbitals are shown by arrows (or half arrows).
  
  Example of sp hybridization of BeCl₂:

     




                    Depicting the hybridization for different steric number

• Steric Number Method
  What is the steric number?
  Steric number is the number of atoms or group of atoms directly bonded to the central atom of a molecule plus the number of lone pairs attached to the central atom.
  
  
• Example: Steric number of Methane, CH₄
   Steric number = 0 + 4
                          = 4

Hence, hybridisation of CH₄ is SP³.

• Formula for steric number
  To calculate the steric number, a mathematical expression used is:

where V = Number of valence electrons in the central atom
M = Number of surrounding monovalent atoms (H or halogens)
q = Charge on the ion
Example: Hybridisation of CO₃^2-
Here central atom is .

Hybridisation involving d-orbitals

In addition to and P orbitals, the elements in the third period have d orbitals. The energy of the 3d orbitals is similar to that of the 3s and 3p orbitals. 3d orbitals have an energy that is comparable to that of 4s and 4p orbitals. As a result, hybridization including 3s, 3p, and 3d, as well as 3d, 4s, and 4p, is possible.

          Hybridisation of Pcl₅ is SP₃d.

• SP₃d² hybridization: 
  Formation of SF₆

         

Hybridisation of SF₆ is SP³d².

Practice problems

Q1. Calculate the     hybridisation of C0₂ using the concept of steric number.
Answer: Steric number = Number of sigma bond pairs + Number of lone pairs



Steric number = 2 + 0 = 2  (since number of sigma bond is 2 and lone pair on C atom is 0)
For steric number equal to 2, the hybridisation comes out to be SP . Carbon dioxide (CO₂) is therefore linear in geometry. The shape of CO₂ is linear because there are no lone pairs affecting the orientation of the molecule.

Q2. The carbon atoms in acetylene are
A. SP
B. SP²
C. SP³
D. SP³d

Answer: A

The carbon atoms present in acetylene show sp hybridisation.
Using steric number, it can be easily calculated.
Steric number = Number of sigma bond pairs + Number of lone pairs

                         = 2 + 0
                         = 2
Hence, acetylene shows sp hybridisation.

Q3. The geometry of PlCO₂ is  
A. Tetrahedral
B. Trigonal planer
C. Square planer
D. Linear

Answer: B. 

By using steric number calculation we can predict the hybridisation and then the geometry of PlCO₂.
Steric number = Number of sigma bond pairs + Number of lone pairs

Steric number = 3 sigma bond pairs + 0 lone pairs (Having 2 pi bonds)
                       = 3 
The hybridisation comes out to be SP²
The geometry of PlCO₂ is trigonal planar. Two oxygen atoms and one chlorine atom are placed at

the corners of an equilateral triangle.

Q4. What is the hybridisation of XeF₄?
A. SP
B. SP²
C. SP³
D. SP³d²

Answer: D.
Solution:  Using this formula of Steric Number = , we can calculate the hybridisation of XeF₄. 
Number of valence electrons on the Xe atom (V) = 8
Number of surrounding monovalent atoms (M) = 4
There is no charge, q = 0

Steric Number = 6 means the hybridisation is SP³d².

Frequently asked questions-FAQs

Q1. What do you mean by orbital overlapping? 

Answer: When two atoms are so close together, their atomic orbitals partially overlap. This partial merging of atomic orbitals is called orbital overlapping.

Q2. What are the different d-orbitals in d subshell?

Answer:  There are 5 d-orbitals in d subshell. These are

Q3. What is the difference between geometry and shape?

Answer: Geometry of a molecule is the arrangement of orbitals containing lone pair and bond pair electrons around a central atom. Shape or structure is the arrangement of atoms around the central atom excluding the lone pairs.

Q4. Why does ammonia have a distorted tetrahedral geometry in spite of having sp³ hybridisation?

Answer: Ammonia has a distorted tetrahedral geometry and trigonal pyramidal shape. This is due to the presence of the lone pair that exerts greater lone pair-bond pair repulsion on the bonding orbitals. The bond angle in ammonia is 107°, which is less than the standard 109.5° for a tetrahedral molecule having sp³.

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