Bond Order
Definition and meaning of Bond Order in chemistry.
Bond order is a measure of the number of chemical bonds between two bonded atoms, calculated in molecular orbital (MO) theory as half the difference between the number of electrons in bonding and antibonding molecular orbitals.
In more detail
A higher bond order generally means a shorter, stronger bond, since more net bonding electron density holds the atoms together. Bond order need not be a whole number: in species with delocalized electrons, such as resonance hybrids or aromatic rings, it is an average over the contributing structures. For simple Lewis structures, bond order can also be found by simply counting the number of shared electron pairs between two atoms (one pair = single bond, two pairs = double bond, and so on).
Key facts
| Field | Physical Chemistry |
|---|---|
| Formula (used in MO theory) | Bond order = (bonding e⁻ − antibonding e⁻)/2 |
| Classic example | N2: bond order = 3 (triple bond) |
| Trend | Higher bond order → shorter, stronger bond |
For N2, MO theory gives 8 electrons in bonding orbitals and 2 in antibonding orbitals, so bond order = (8 − 2)/2 = 3, matching its known triple bond, very short N–N distance (110 pm), and very high bond dissociation energy (~945 kJ/mol).
Frequently asked questions
Can bond order be a fraction?
Yes. Delocalized systems give non-integer values, e.g., each C–C bond in benzene has a bond order of 1.5, and O2's MO description gives a bond order of 2 with two unpaired electrons, explaining its paramagnetism.
What does a bond order of zero mean?
A bond order of zero means there is no net bonding, so the species is not expected to exist as a stable molecule, as in He2, where bonding and antibonding electron counts cancel out.