Antibonding molecular orbits (MOs) are normally higher in energy than bonding MOs. They are occupied by two electrons at a time and (in the case of hydrogen), each atom can contribute only one electron, therefore only the s (bonding) MO is occupied and the H2 molecule is more stable than two separate H atoms.
A molecular orbital becomes antibonding as there is actually less electron density between the two nuclei than there would be if there was no bonding interaction at all. When an MO changes sign (from positive to negative) between two atoms, it is said to be antibonding with respect to those atoms. Antibonding orbitals are often labelled with an asterisk (*) on molecular orbital diagrams.
In molecules with several atoms, such as benzene, a particular MO may be bonding with respect to some adjacent pairs of atoms and antibonding with respect to other pairs. If the bonding interactions outnumber the antibonding interactions, the MO is said to be "bonding," while if the antibonding interactions outnumber the bonding interactions, the MO is said to be "antibonding". Since each carbon atom contributes only one electron to the π-system of benzene, there are six π-electrons and therefore only the three lowest-energy MOs (the bonding ones) are filled.
Another particular feature of antibonding is that the antibonding orbital is more antibonding than the bonding orbital is bonding. This leads to the conclusion that the energy of both MOs are raised by the presence of nucleus-nucleus repulsion.
