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 1s_¹. 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 1s_², 2s_², 2p_⁵. 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 1s_², 2s_², 2p_⁵. 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 ( л ) 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.

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To identify sigma bond and Pi bond easily:

Watch Now !!!

Easiest Trick to Identify Sigma bond and pi bond

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