why overlapping of sigma bond is stronger compared to the pi bond?
There are essentially two basic ways that orbitals can overlap with each other to form a bond. Lets consider two atoms with one p-orbital each that will overlap to form a bond. The orbitals can either bond head-on, so that they touch only once, or they can bond side-by-side, touching in two places. The first type, the head-on bond, is called a sigma-bond (usually denoted with the lowercase Greek letter sigma, ?), and the second type, the side-by-side bond, is called a pi-bond (denoted with ?). In the figures below, an example of the sigma bond is on the left and the pi-bond is on the right.