Pi Bonds

Chemical bonds are forces which keep atoms joined together. Chemical bonds are classified into covalent bond, coordinate bond, ionic bond and hydrogen bond. Covalent bonds are those bonds which are formed by sharing of electrons between two atoms. It is also known as molecular bond. Here in this article we are going to discuss sigma and pi bonds which are covalent bonds only but are formed by different types of overlapping between orbitals. Here we will discuss pi bonds in detail and will have a short look at sigma bond and difference between pi and sigma bond. 

What are Pi Bonds? 

The covalent bond which is formed by lateral overlapping of the half-filled atomic orbitals (p - orbitals) of atoms is called pi bond. It is denoted by . We find pi bonds in alkenes and alkynes. The electrons which take part in the formation of pi covalent bonds are called pi – electrons.  Formation of pi bond is given below between the two orbitals – 

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Characteristics of Pi Bond 

  • Pi bond is formed by sideways overlapping of two parallelly oriented pi orbitals of adjacent atoms. 

  • In pi bonds overlapping takes place at the side of the of the two lobes of p – orbitals so the extent of overlapping of less than sigma bond. Hence, pi bonds are weaker than sigma bonds. 

  • In pi bonds, electron density is concentrated in the region perpendicular to the bond axis. 

  • The molecular orbital of pi bond is oriented above and below the plane containing the nuclear axis. 

  • All atoms of the molecule must be in the same plane, if pi bond is formed in the molecule. 

  • In general, double covalent bonds consist of one pi and one sigma bond while triple covalent bonds consist of one sigma and two pi – bonds. Here you need to note that if only one covalent is present between atoms then it will always be a sigma covalent bond. 

Formation of Pi Bond in Oxygen Molecule 

Oxygen molecules are formed by joining two oxygen atoms covalently. Each oxygen atom has a total of 8 electrons. By writing its electronic configuration we can see each oxygen atom has two p – orbitals which have only one electron in them in the valence shell. 

The electronic configuration of oxygen atom –

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In the above figure, you can see 2py, 2pz orbitals of the valence shell of oxygen atom is singly occupied. When two oxygen atoms approach each other to form an oxygen molecule, one set of singly occupied p – orbitals get head on overlapped axially and form a sigma bond. While the other set of singly occupied p – orbitals get sideways overlapped and form a pi bond. Thus, in an oxygen molecule one sigma and one pi bond is formed. 

What is Sigma Bond? 

The strongest covalent bond which is formed by the head on overlapping of the atomic orbitals is called sigma bond. It is denoted by . We find sigma bonds in alkanes, alkenes, alkynes. Formation of sigma bond is given below between the orbitals- 

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As you can see above figure sigma bond is formed by following three types of overlapping between orbitals –

  • S – S Overlapping – s- orbital is spherical in shape and whenever they reach a point of maximum attraction, they overlap and form a sigma bond. This can be understood clearly by taking an example of a hydrogen molecule. Hydrogen molecule is formed by two hydrogen atoms. Each hydrogen atom contains one s – orbital in the valence shell which is singly occupied. When two hydrogen atoms approach each other to form a hydrogen molecule, these two singly occupied s – orbitals of valence shell combine with each other and form a molecular orbital or sigma bond. 

Electronic configuration of H atom –

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Formation of sigma bond in hydrogen molecule –

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  • S – P Overlapping – Sigma bond is formed by head on overlapping of s – orbital and p – orbital. It can be understood by formation of hydrogen fluoride molecules. In the formation of hydrogen fluoride molecule, singly occupied 1s – orbital of valence shell of hydrogen get head on overlapped with singly occupied p – orbital of valence shell of fluorine and forms a sigma bond. 

Electronic configuration of H atom –

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Electronic configuration of F atom –

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Formation of sigma bond in HF molecule –

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  • P – P Overlapping – Overlapping of p - orbitals form both pi bond and sigma bond. When p – orbitals are overlapped sidewise or lateral, that time pi bond is formed as discussed initially. While when the bonds are formed by the head on overlapping of p – orbitals are called sigma bonds. It can be illustrated by the formation of fluorine molecules. For the formation of fluorine molecules two singly occupied 2pz orbitals of valence shell of fluorine atom get overlap and form molecular orbitals or sigma bonds. 

Electronic configuration of F atom –

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Formation of sigma bond in fluorine molecule –

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Difference Between Pi and Sigma Bonds

S. No. 

Sigma Bond 

Pi Bond 

1. 

Covalent bond which is formed by the head on overlapping of the atomic orbitals is called sigma bond.

Covalent bond which is formed by lateral overlapping of the half-filled atomic orbitals of atoms is called pi bond.

2. 

It is the strongest covalent bond. 

It is weaker than the sigma bond. 

3. 

It denoted by

It is denoted by

4. 

In sigma bonds, overlapping orbitals can be pure orbitals, hybrid orbitals and one hybrid and one pure orbital. 

In pi bond, overlapping orbitals are always pure orbitals only. Pure orbitals are unhybridized orbitals. 

5. 

It can exist independently. Example -alkane 

It can exist with a sigma bond only. Example- alkene and alkyne. 

6. 

It allows free rotation of orbitals. 

It restricts free rotation of orbitals. 

7. 

Atoms with sigma bonds are highly reactive. 

Atoms with pi bonds are less reactive than atoms having sigma bonds only. 

8. 

It has cylindrical charge symmetry around the bond axis. 

There is no symmetry in pi bonds. 

9. 

It determines the shape of the molecule. 

It doesn’t determine the shape of the molecule. 

10. 

Example – CH4  

Example – C2H4  


This ends our coverage on the topic “Pi bonds”. We hope you enjoyed learning and were able to grasp the concepts. If you are looking for solutions to NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.