Bond order measures the number of bonding electron pairs in a molecule and is used to judge the strength of molecular bonds. It is calculated by subtracting the number of antibonding electrons from the number of bonding electrons and dividing the sum by two. Bond orders have practical applications in chemistry, including in the creation of superalloys. Molecular orbital theory provides a simple method for calculating bond order, and an order of 3 indicates a very strong bond. Highly stable bond orders are usually very long covalent bonds, such as those found in diamond.
A bond order is the measure of the number of bonding electron pairs in a molecule. It is used as a relativistic index to judge the strength of molecular bonds. While calculation methods vary, the order of bonds 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 innovative chemicals can use bond order calculations to judge the relative stability of the molecules they create. Most notably, perhaps, bond orders were used in the creation of superalloys, such as single crystal-based nickel.
Bond orders have always been a big part of chemistry; the most modern understanding of bond order, however, has been made possible by molecular orbital theory. This theory, pioneered 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 principles of molecular bonding theory which stated that bonding orbitals strengthen bonds, while antibonding orbitals weaken bonds, in equal proportions. The theory also described how orbitals closest to nuclei are unable to influence the strength of the bond, which has contributed to a broader perspective into quantum mechanics that previous theories were unable to obtain.
Calculating bond orders using molecular orbital theory is quite simple. The bond order is equal to the number of bonding electrons minus the number of antibonding electrons and that sum is divided by two. To find the number of bonding and anti-bonding electrons, an electron configuration can be used, taking into account sigma and pi bonds.
The interpretation of the index number is also critical. An order of zero indicates that the bond is volatile. A bond order of one indicates a stable bond and a bond order of 2 indicates that a bond does not break easily. Bonds with an order of 3 are considered very strong. Highly stable bond orders are usually very long covalent bonds. For example, diamond – one of the strongest natural substances on earth – is made entirely of carbon and has a very long bond length of 154 picometers, or 154 trillionths of a metre. Since diamond consists entirely of carbon and the bonds between multiple carbon atoms are almost always double bonds – bond order 2 – it is naturally very strong.
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