Distinguish between a sigma and a pi bond.

The strongest covalent bond which is formed by the head-on overlapping of atomic orbitals is called the sigma bond.  It allows free rotation of bonds.

Example: CH₃-CH₃

The covalent bond which is formed by lateral overlapping of atomic orbitals of atoms is called pi bond. It doesn’t allow free rotation of bonds.

Example: CH₂=CH₂

Final answer: Covalent bond which is formed by the head-on overlapping of atomic orbitals is called the sigma bond andcovalent bond which is formed by lateral overlapping of atomic orbitals of atoms is called pi bond.