What is Bond Order?

A bond order is the measure of the number of bonding electron pairs in a molecule. It is used as a relativistic index for judging the strength of molecular bonds. Although the methods of calculation vary, bond order is almost always a number between 1 and 3, with 3 being the strongest. Bond orders have a variety of practical applications in the field of Chemistry. Manufacturers of novelty chemicals can use bond-order calculations to judge the relative stability of the molecules they create. Most notably perhaps, bond orders have been used in the creation of super alloys, such as single crystal-based nickel.

Bond orders have always been an important part of chemistry; the most modern understanding of bond order, however, was made possible by the Molecular Orbital Theory. This theory, introduced by scientists Frederich Hund, Robert Mulliken, John C. Slater, and John Leonard-Jones, was the first to accurately describe simple and elegant calculations for determining bond orders. The calculations were derived from the essential tenants of molecular bond theory which established that bonding orbitals strengthen bonds, while anti-bonding orbitals weaken the bonds, in equal proportions. The theory also described how orbitals closest to the nuclei are incapable of influencing bond strength, which contributed to a grander perspective in quantum mechanics that previous theories were not able to achieve.

Calculating bond orders using Molecular Orbital Theory is fairly basic. Bond order equals the number of bonding electrons minus the number of anti-bonding electrons, and that sum is divided by two. To find the number of bonding and anti-bonding electrons, one can use an electron configuration, accounting for sigma and pi bonds.

Interpreting the index number is basic as well. A bond order of zero indicates that the bond is unstable. A bond order of one indicates a stable bond, and a bond order of 2 indicates that a bond is not easily broken. Bonds with an order of 3 are considered very strong. Highly stable bond orders are usually very long, covalent bonds. For instance, diamond — one of the strongest natural substances on the earth — is entirely made of carbon and has very long bond length of 154 picometers, or 154 trillionths of a meter. Since diamond is made purely of carbon, and bonds between multiple carbon atoms are almost always double bonds — bond order 2 — it is naturally very strong.