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Concept of Sigma and Pi bond

Sigma Bond (σ)

↪ When the overlap of orbital’s of two atoms takes place along the line joining the two nuclei (orbital axis) then the covalent bond formed is called sigma (σ) bond.
↪ These bonds can be formed due to ’s-s', 's-p' or 'p-p' overlap along the orbital axis. Free rotation around a sigma bond is always possible.

Fig: Formation of sigma bond due to various overlapping

Pi bond (∏)

↪ When the two atoms overlap due to the sideways overlap of their 'p' orbitals, the covalent bond is called as pi (∏) bond.
↪ In a pi bond the electron density is concentrated in the region perpendicular to the bond axis. 

Characteristics of the pi bonds                  

↪ The pi bonds are weak because the orbital overlap is partial.

↪ For a complete sideways overlap the 'p' orbitals should be parallel to each other. This is possible when all the atoms of the molecule are in the same plane i.e., there is no rotation of one part of the molecule relative to the other about the pi bond.

Fig: Rotation about a double bond

↪ In molecules containing double bond, which has a pi bond, there is no free rotation and such molecules exist in isomeric forms of 'cis' and 'trans'.
↪ In the 'cis' form the similar atoms lie on the same side of a plane placed along the internuclear axis while in the 'trans' form the similar atoms lie on the opposite sides.

↪ The electrons in the pi bond are placed above and below the plane of the bonding atoms and so they are more exposed.
↪ They are more susceptible to attack by electron seeking or oxidizing agents.
↪ Hence, they are the most reactive centers in unsaturated (multiple bonded) compounds.

Comparison between sigma bond and Pi bond

Sigma BondPi Bond
↪ Formed due to the axial overlap of two orbitals.
↪ Formed by the lateral overlap of two pi orbitals.
↪ Only one sigma bond exists between two atoms.
↪ There can be more than one pi bonds between two atoms.
↪ The electron density is maximum and cylindrically symmetrical about the bond axis.
↪ The electron density is high along the direction at right angles to the bond axis.
↪ Free rotation about the sigma bond is possible.↪ Free rotation about the pi bond is not possible.
↪ This bond can be independently formed i.e., without the formation of a pi bond.↪ The pi bond is formed after the sigma bond has been formed.
↪ Sigma bond is a relatively strong bond.↪ Pi bond is a weak bond.