What is "bond order"

n. (context chemistry English) The number of bonds between a pair of atoms in a molecule

Bond order is the number of chemical bonds between a pair of atoms. For example, in diatomic nitrogen N≡N the bond order is 3, in acetylene H−C≡C−H the bond order between the two carbon atoms is also 3, and the C−H bond order is 1. Bond order gives an indication of the stability of a bond. The element with bond order value 0 cannot exist. Those with fractional values exist but are not stable.

In molecules that have resonance or nonclassical bonding, bond order does not need to be an integer. In benzene, where the delocalized molecular orbitals contain 6 pi electrons over six carbons essentially yielding half a pi bond together with the sigma bond for each pair of carbon atoms, giving a calculated bond order of 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.