is the number of bonds between a pair of atoms. For example in nitrogen
N≡N the bond order is 3, in acetylene
H−C≡C−H the bond order between the two carbon atoms is 3 and the C−H bond order is 1. Bond order gives an indication to the stability of a bond. In a more advanced context, bond order need not be an integer. A good example of this is bonds between carbon in the molecule benzene
where the delocalized molecular orbitals contain 6 pi electrons
over six carbons essentially yielding half a pi bond
. Together with the sigma bond
the bond order is 1.5. Furthermore bond orders of 1.1 for example can arise under complex scenarios and essentially refer to bond strength relative to bonds with order 1.
In molecular orbital theory
bond order is also defined as the difference, divided by two, between the number of bonding electrons
and the number of antibonding electrons
per the equation below. This often but not always yields the same result. Bond order is also an index of bond strength
and is also used extensively in valence bond theory
For example, the bond order concept is used in molecular dynamics bond order potentials.
The magnitude of the bond order is associated with the bond length. According to Pauling, the bond order is experimentally described by:
Where is the bond length experimentally measured, is the single bond length, and b is a constant, depending on the atoms. A good approximation for b use to be 0.37.