p z orbital of the other atom but not with the p x or p y orbitals because of their different symmetries. . The combining atomic orbitals must overlap to the maximum extent . Greater the extent of overlap, the greater will be the electron-density between the nuclei of a molecular orbital.
. . Types of Molecular Orbitals Molecular orbitals of diatomic molecules are designated as σ (sigma), π (pi), δ (delta), etc. In this nomenclature, the sigma ( ) molecular orbitals are symmetrical around the bond-axis while pi ( ) 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 σ 1s and σ * 1s [Fig. . (a), page ].
If internuclear axis is taken to be in the z-direction, it can be seen that a linear combination of 2p z - orbitals of two atoms also produces two sigma molecular orbitals designated as 2p z and * 2p z . [Fig. . (b)] Molecular orbitals obtained from 2p x and 2p y 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 * [Fig. . (c)]. 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. . . Energy Level Diagram for Molecular Orbitals We have seen that s atomic orbitals on two atoms form two molecular orbitals designated as σ s and σ * s .
In the same manner, the s and p atomic orbitals (eight atomic orbitals Fig. . Contours and energies of bonding and antibonding molecular orbitals formed through combinations of (a) 1s atomic orbitals; (b) 2p z atomic orbitals and (c) 2p x atomic orbitals. on two atoms) give rise to the following eight molecular orbitals: Antibonding MOs σ ∗ 2s