Electrons in bonding molecular orbitals
stabilize the molecule.
Electrons in antibonding molecular orbitals
destabilize the molecule.
A bond forms only if the bonded atoms are more stable than the separated atoms. Therefore for a molecule to exist there must be
more electrons in its
bonding molecular orbitals
than in its
antibonding molecular orbitals.
The
net number of
bonding electrons for a particular species equals the number of
electrons occupying
bonding orbitals
minus the number of
electrons occupying
antibonding orbitals.
The
bond order is the number of net bonding electrons divided by two because each bond involves an electron pair. Hydrogen (H
2) has two electrons in a bonding molecular orbital and none in antibonding orbitals; therefore the bond order in H
2 is 1.
If the bond order is 0, no bond forms.
A bond order of 1 corresponds to a single bond, 2 to a double bond, etc.
Bond orders can also be fractional (½, 1½).