Valence Bond (VB) Theory Notes

Valence Bond (VB) Theory:

Valence Bond Theory Class 11 Chemistry (vbt )

This theory has the following postulates:

define covalent bond
  • 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:

define sigma 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.

orbital overlapping
Different types of head-on overlapping

Sigma Bond Example:

methane ch4
methane

Pi ( л ) Bond:

define pi bond
pi bond formation

            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:

Difference between sigma bond and Pi Bond
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.

ethene c2h4
ethene
ethene
orbital structure of ethene
c2h2
ethyne
c2h2
orbital structure of ethyne
sigma bonds in methane

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)

Molecular orbital theory

How to draw Molecular orbital Diagram for Fluorine

How to draw molecular orbital diagram for nitrogen

How to draw molecular orbital diagram for oxygen atom / ion

How to draw molecular orbital diagram for hydrogen and helium

Difference between the Molecular orbital structure of ethene and ethyne

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