Editing Molecular orbital theory (section)

==Bond order==

[[File:Molecular orbital diagram of He2.png|thumb|Molecular orbital diagram of He<sub>2</sub>]]

Bond order is the number of chemical bonds between a pair of atoms. The bond order of a molecule can be calculated by subtracting the number of electrons in [[Antibonding molecular orbital|anti-bonding]] orbitals from the number of [[bonding molecular orbital|bonding]] orbitals, and the resulting number is then divided by two. A molecule is expected to be stable if it has bond order larger than zero. It is adequate to consider the [[valence electron]] to determine the bond order. Because (for [[principal quantum number]] ''n'' > 1) when MOs are derived from 1s AOs, the difference in number of electrons in bonding and anti-bonding molecular orbital is zero. So, there is no net effect on bond order if the electron is not the valence one.

<math>\text{Bond order} = \frac12 (\text{Number of electrons in bonding MO} - \text{Number of electrons in anti-bonding MO})</math>

From bond order, one can predict whether a bond between two atoms will form or not. For example, the existence of He<sub>2</sub> molecule. From the molecular orbital diagram, the bond order is <math display=inline>\frac12(2-2)=0</math>. That means, no bond formation will occur between two He atoms which is seen experimentally. It can be detected under very low temperature and pressure molecular beam and has [[binding energy]] of approximately 0.001 J/mol.<ref>{{cite book |last1=Miessler |first1=Gary L. |title=Inorganic Chemistry |last2=Fischer |first2=Paul J. |last3=Tarr |first3=Donald A. |date=2013-04-08 |publisher=Pearson Education |isbn=978-0-321-91779-9 |language=en |url=https://books.google.com/books?id=VSktAAAAQBAJ}}</ref> (The [[helium dimer]] is a [[van der Waals molecule]].)

Besides, the strength of a bond can also be realized from bond order (BO). For example:

For H<sub>2</sub>: Bond order is <math display=inline>\frac12(2-0)=1</math>; bond energy is 436 kJ/mol.

For H<sub>2</sub><sup>+</sup>: Bond order is <math display=inline>\frac12(1-0)=\frac12</math>; bond energy is 171 kJ/mol.

As the bond order of H<sub>2</sub><sup>+</sup> is smaller than H<sub>2</sub>, it should be less stable which is observed experimentally and can be seen from the bond energy.