Molecular orbitals of diatomic molecules are designated as σ (sigma), π (pi), δ (delta), etc.
In this nomenclature, the sigma (σ) molecular orbitals are symmetrical around the bond-axis while pi (π) molecular orbitals are not symmetrical.
For example, the linear combination of 1s orbitals centered on two nuclei produces two molecular orbitals which are symmetrical around the bond-axis.
Such molecular orbitals are of the σ type and are designated as σ1s and σ*1s. If internuclear axis is taken to be in the z-direction, it can be seen that a linear combination of 2pz- orbitals of two atoms also produces two sigma molecular orbitals designated as σ2pz and σ*2pz.
Molecular orbitals obtained from 2px and 2py orbitals are not symmetrical around the bond axis because of the presence of positive lobes above and negative lobes below the molecular plane.
Such molecular orbitals, are labelled as π and π * . A π bonding MO has larger electron density above and below the inter -nuclear axis. The π * antibonding MO has a node between the nuclei.