Chemistry Bond order|bond strength order|all Examples.

Bond order

Hello friends, welcome to our blog,today we are going to discuss the bond order, how to calculate it and what's its formula. The bond order can be calculated with the help of two concepts.
1. Lewis concept and
2. Molecular orbital theory(MOT)

According to Lewis concept , in a covalent bond, the bond order may be defined as:

 The number of bonds between the two atoms in a molecule. 

For example,

Bond order of H2(H−H bond) = 1           ;                  Bond order of O2 (O=O bond) = 2
Bond order of N2(N≡N bond)  =3            ;                 Bond order of (H−C≡C−H)
                                                                                                 carbon-carbon bond order= 3.

1.The Isoelectronic molecules and ions have identical bond orders
    For example, bond order of  N2 , CO and NO+  (all have 14 electrons) is 3.        

2. With the increasing bond order, the bond enthalpy increases and bond level decreases. For example,
 Bond order of N2 =3;Bond enthalpy = 945 KJ mol-1       
Bond order of O2 = 2;Bond enthalpy = 498 KJ mol-1     
Bond order of F2 =1;Bond enthalpy =  158 KJ mol-1
This video helps you how to write electronic configuration using MOT concept.
In case of Molecular orbital theory, the electron present in bonding molecular orbital is known as bonding electrons while electron present in antibonding molecular orbital is called antibonding electrons.

 Also, we have learnt that bonding molecular orbitals have lower energy as compared to antibonding molecular orbitals and greater stability. Thus, the molecular orbital stability  can be predicted in terms of electrons present in bonding molecular orbital( Na electrons)and those present in antibonding molecular( Nb electrons).The following generalisation can be made.


A. If Nb > Na ,the molecule is stable.
B. If Nb < Na ,the molecule is unstable.
C. If Nb = Na ,the molecule is unstable.

In fact, the bonding electrons tend to bring nuclei of the atomic orbital closer while the antibonding electrons tend to  push them apart. Thus, a stable  molecule will be formed only if Nb  electrons are more than Nelectrons.

Bond order Define

Half  of the difference between the number of electrons present in bonding and antibonding molecular orbitals.

Bond order formula

Thus,Bond order(B.O)=1/2[Nb -Na ] 
The Bond order may be whole number,fractional or even zero..It may also be positive or negative.    
If Bond Order = Positive, the molecule is stable.
If Bond Order = zero, the molecule is unstable or not exists.   
If Bond Order = Negative, the molecule is unstable.               

Hydrogen molecule(H2 )    

Molecular orbital electronic configuration of H2 =[ s1s ]2  [ s1s* ]0
Bond Order :-1/2[Nb -Na] = 1/2[ 2 -0 ] = 1    
Positive value indicate =stable
It is diamagnetic in nature since the only occupied molecular orbital filled.

Hydrogen molecule ion(H2 +

Total no. of electrons = 1
Molecular orbital electronic configuration of H2 + =[ s1s ]1  [ s1s* ]0
Bond Order :-1/2[Nb -Na] = 1/2[ 1 -0 ] = 1/2
An unpaired electron always indicates that the molecule is paramagnetic.

He2+ molecule ion, 

Total no. of electrons = 3,
Molecular orbital electronic configuration of He2+  cation =[ s1s ]2  [ s1s* ]1
Bond Order :-1/2[Nb -Na] =1/2[ 2 -1 ] = 1/2
Unpaired electron, so paramagnetic.

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