what are sigma and pi bonds

A covalent bond established between two atoms having the maximum density of the electron cloud along the line connecting the centre of the bonded atoms is called aσ-bond.Aσ-bond is thus said to possess a cylindrical symmetry along the internuclear axis.

Let us now consider the combination of two nitrogen atoms. Of the three singly occupied p-orbitals in each, only one p-orbital from each nitrogen (say, the pxmay undergo “head –on” overlap to form as-bond. The other two p-orbitals on each can no longer enter into a direct overlap. But each p-orbital may undergo lateral overlap with the corresponding p-orbital on the neighbour atom. Thus we have two additional overlaps, one by the two pyorbitals, and the other by the two pzorbitals. These overlaps are different from the type of overlap in as-bond.

For each set of p-orbitals, the overlap results in accumulation of charge cloud on two sides of the internuclear axis. The bonding electron cloud does no more posses an axial symmetry as with thes-bond; instead, it possess a plane of symmetry. For the overlap of the pzatomic orbital, the xy plane provides this plane of symmetry; for the overlap of the pyatomic orbitals, the zx plane serves the purpose. Bonds arising out of such orientation of the bonding electron cloud are designated asπ-bonds.

The bond formed by lateral overlap of two atomic orbitals having maximum overlapping on both sides of the line connecting the centres of the atoms is called aπ-bond. Aπ-bond possess a plane of symmetry, often referred to as the nodal plane.