Valence Bond (VB) Theory:
This theory has the following postulates:

- Covalent bonds are formed by overlapping atomic orbitals.
- Electron density is high in the overlapped area and is thus responsible for holding atoms together.
- Only half-filled atomic orbitals take part in covalent bond formation.
- The half-filled atomic orbitals must contain electrons with opposite so they cancel the magnetic lines of force and can live together.
- By the overlapping of orbitals, two types of bonds are formed sigma bonds and pi bonds.
- Atomic orbitals of different shapes and energies sometimes are mixed to form hybrid orbitals.
Sigma ( δ ) Bond:

When half-filled atomic orbitals are overlapped on the bond axis the overlapping is called sigma overlapping and the bond formed is called a sigma bond. Sigma bond is formed due to s-s or s-p or p-p overlapping. As sigma overlapping is larger so sigma bond is very strong.
s-s Overlapping:
The sigma bond which is formed between Hydrogen atoms is due to s-s overlapping.
The electronic configuration of Hydrogen is 1s1. The half-filled s-orbital of one Hydrogen atom overlaps with half filled s-orbital of another Hydrogen atom on the bond axis. So H ─ H bond is sigma bond.
s-p Overlapping:
The sigma bond which is formed between a hydrogen atom and fluorine atom is due to s-p overlapping.
The electronic configuration of hydrogen is 1S1 and that of Fluorine is 1s2, 2s2, 2p5. The half-filled s-orbital of the Hydrogen atom overlaps with the half-filled p-orbital of the Fluorine atom on the bond axis. So H ─ F bond is a sigma bond.
p-p Overlapping:
The sigma bond which is formed between Fluorine atoms is due to p-p overlapping.
The electronic configuration of Fluorine is 1s2, 2s2, 2p5. The half-filled p-orbital of one Fluorine atom overlaps with the half-filled p-orbital of another Fluorine atom on the bond axis. So F ─ F bond is a sigma bond.

Sigma Bond Example:

Pi ( Đ» ) Bond:

When half-filled atomic orbitals are overlapped away from the bond axis (side-wise or lateral overlapping) the overlapping is pi overlapping and the bond formed is called a pi bond. A pi bond is formed due to the lateral overlapping of two parallel p-orbitals. As pi overlapping is little so pi bond is a weak bond. e.g Oxygen molecule has one sigma bond and one pi bond.
The electronic configuration of oxygen atom is 1S2, 2S2, 2P4. Thus oxygen has two half-filled p-orbitals.
One of the half-filled p-orbital of one oxygen atom overlaps with a half-filled p-orbital of another oxygen atom on the bond axis thus a sigma bond is formed. The half-filled p-orbitals of two oxygen atoms which are parallel to each other are side wisely or laterally overlapped and form a pi bond. Thus a double bond is formed between two oxygen atoms consisting of sigma and a pi bond.
Difference between sigma bond and Pi Bond:

Examples of Sigma bond and Pi Bond:
A few examples of sigma and pi bonds are as follow.
The simplest trick to identifying the sigma bond and pi bond is:
All single covalent bonds are sigma bonds. e.g methane CH4
In a double covalent bond, the First bond is the sigma bond and the second bond is always the pi bond e.g. in ethene C2H4
In the case of a triple covalent bond, the first bond is the sigma bond and the second and third bonds are the pi bonds e.g. in ethyne
( acetylene) C2H2
Sigma bond and Pi bond Calculation:
The below diagrams shows the total number of sigma bonds and pi bonds around the carbon atoms.





To identify sigma bond and Pi bond easily:
Watch Now !!!
Easiest Trick to Identify Sigma bond and pi bond
Most Important Recommended videos ( in Urdu / Hindi) :
Valence Shell Electron Pair Repulsion Theory (vsepr)
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How to draw molecular orbital diagram for oxygen atom / ion
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Difference between the Molecular orbital structure of ethene and ethyne
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