Bond Order
CHEM 121
What is meant by “bond order?”
It is simply the number of bonds between two atoms.
Single bond has a bond order = 1
Double bond has a bond order = 2
Triple bond has a bond order
= 3
The complexity arises when we examine a structure with resonance structures.
The nitrate ion (NO3–) has the resonance structures and resonance hybrid as shown:
_
.. + ..
:O N O
.. :
:O
.. :_
_
_
_
.. + ..
:O
..:
.. N O
:O
..
_
 .. + . .. 
: O... .. O..
.N
..
..O..

_
.. + ..
:O
.. N O :
:O
.. : _
_
_
_
We say that the bond order between N and O is more than one because sometimes there is a double
bond there.
How often is the double bond there?
Between each N and O, it is there 1/3 of the time (in one structure out of three shown above).
Therefore, the bond order between N and O is 1⅓.
For the nitrite ion (NO2–)
_ ..
+
:O
.. N
:O
..
..
_
+
:O
..
the resonance structure is...
..
N
:O
.. :_
What is the bond order for between N and each O?
Ans. 1½
_
..
: O... + ..
N.
..
O
: :
