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| '''Bond order''' is the number of [[chemical bond]]s between a pair of [[atom]]s. 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 to the stability of a bond.
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| In molecules that have [[resonance (chemistry)|resonance]] or nonclassical bonding, bond order does not need to be an integer. In [[benzene]], where the delocalized [[molecular orbital]]s contain 6 [[pi electron]]s 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.
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| In [[molecular orbital theory]], bond order is also defined as the difference, divided by two, between the number of [[bonding electron]]s and the number of [[antibonding electron]]s 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]].
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| :<math>\text{B.O.} = \frac{\text{number of bonding electrons} - \text{number of antibonding electrons}}{2}\ </math>
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| Bond orders of one-half can be stable, as shown by the stability of {{chem|H|2|+}} (bond length 106 pm, bond energy 269 kJ/mol) and {{chem|He|2|+}} (bond length 108 pm, bond energy 251 kJ/mol).<ref>Bruce Averill and Patricia Eldredge, ''Chemistry: Principles, Patterns, and Applications'' (Pearson/Prentice Hall, 2007), 409.</ref>
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| The bond order concept is used in [[molecular dynamics]] and [[bond order potential]]s. The magnitude of the bond order is associated with the [[bond length]]. According to [[Linus Pauling]] in 1947, the bond order is experimentally described by:
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| :<math> s_{ij} = \exp{\left[ \frac{d_{1} - d_{ij}}{b} \right]} </math>
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| where <math>d_{1}</math> is the single bond length, <math>d_{ij}</math> is the bond length experimentally measured, and b is a constant, depending on the atoms. Pauling suggested a value of 0.353 Å for b. The above definition of bond order is somewhat ad hoc and only easy to apply for [[diatomic]] molecules. A standard [[quantum mechanical]] definition for bond order has been debated for a long time.<ref>[[IUPAC Gold Book]] [http://goldbook.iupac.org/BT07005.html ''bond order''] - [http://www.iupac.org/goldbook/BT07005.pdf PDF]</ref>
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| ==References==
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| {{Reflist}}
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| {{Chemical bonds}}
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| {{DEFAULTSORT:Bond Order}}
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| [[Category:Chemical bond properties|Order]]
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