Bond order

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. An element with bond order value 0 cannot exist, but compounds can have a 0 bond value. Isoelectronic species have same bond number.

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.

In molecular orbital theory, bond order is also defined as half the difference between the number of bonding electrons and the number of antibonding electrons as 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.

Generally, the higher the bond order, the stronger the bond. Bond orders of one-half may be stable, as shown by the stability of H+
2 (bond length 106 pm, bond energy 269 kJ/mol) and He+
2 (bond length 108 pm, bond energy 251 kJ/mol).

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