or

Sigma bond is formed due to the overlapping of orbitals along their intermolecular axis. This leads to the effective overlapping and high electron density along intermolecular axis.

On the other hand, a pi bond is formed due to the sideways overlapping of orbitals. This side-ways overlap is partial and results in low electron density in intermolecular axis.

This is why, a sigma bond is stronger than pi bond.

The reason for Sigma bonds to be stronger than Pi bonds is simple.

Sigma bonds are formed by Head-on overlapping of orbitals which results in high electron density in intermolecular axis. And Pi bonds are formed by Sideways overlapping of orbitals which results in less electron density in intermolecular axis.

This causes Sigma bond to be stronger than Pi bond.

Sigma bond is stronger than Pi bond because of the following reasons:

  1. The extent of overlapping in Sigma bond is greater than in Pi bond.
  2. The electron density in intermolecular axis is more in Sigma bond than in Pi bond.
  3. There is free rotation of atom about Sigma bond but not about Pi bond.