the nuclei of the bonded atoms because of which the repulsion between the nuclei is very less while in case of an antibonding molecular orbital, most of the electron density is located away from the space between the nuclei. Infact, there is a nodal plane (on which the electron density is zero) between the nuclei and hence the repulsion between the nuclei is high. Electrons placed in a bonding molecular orbital tend to hold the nuclei together and stabilise a molecule. Therefore, a bonding molecular orbital always possesses lower energy than either of the atomic orbitals that have combined to form it.
In contrast, the electrons placed in the antibonding molecular orbital destabilise the molecule. This is because the mutual repulsion of the electrons in this orbital is more than the attraction between the electrons and the nuclei, which causes a net increase in energy. It may be noted that the energy of the antibonding orbital is raised above the energy of the parent atomic orbitals that have combined and the energy of the bonding orbital has been lowered than the parent orbitals. The total energy of two molecular orbitals, however, remains the same as that of two original atomic orbitals.
. . Conditions for the Combination of Atomic Orbitals The linear combination of atomic orbitals to form molecular orbitals takes place only if the following conditions are satisfied: . The combining atomic orbitals must have the same or nearly the same energy .
This means that s orbital can combine with another s orbital but not with s orbital because the energy of s orbital is appreciably higher than that of s orbital. This is not true if the atoms are very different. . The combining atomic orbitals must have the same symmetry about the molecular axis .
By convention z-axis is taken as the molecular axis. It is important to note that atomic orbitals having same or nearly the same energy will not combine if they do not have the same symmetry. For example, p z orbital of one atom can combine with