Bond Order

The bond order is a measure of the number of electron pairs used in bonding. It is related to the number of electrons in bonding orbitals (N_b) and the number in anti-bonding orbitals (N_a) by the equation     OR

It is number of bonds formed between two atoms in a covalent compound.

Bond Order

 

Where, N_b is number of electrons present in Bonding Molecular orbitals (BMO) and  N_a is number of electrons present in Anti- Bonding Molecular orbitals (ABMO). 

In bond order calculations we have to consider the molecular orbital configuration of the molecule which is represented as Bonding Molecular orbitals (BMO) and Anti- Bonding Molecular orbitals (ABMO). The sequence of molecular orbitals is available in two categories,

 Category 1:- From H₂ to N₂: the sequence of filling electron in molecular orbital is

s 1s < s *1s < s 2s < s *2s < p 2pz = p 2py < s 2px < p *2pz = p *2py < s *2px

 

Category 2:- From O₂ & F₂: the sequence of filling electron in molecular orbital is

s 1s < s *1s < s 2s < s *2s < s 2px < p 2pz = p 2py < p *2pz = p *2py < s *2px

 s *,  p *  = antibonding molecular orbital

s, p  = bonding molecular orbital

To understand bond order let’s take example of

1)    Hydrogen Molecule H₂

      Hydrogen atom has one electron and the H₂ molecule will have 2 electrons.

      Its Configuration: s  (1s)² s  * (1s)⁰

 

 

 

 

 

2) Beryllium Molecule Be₂ 

Be (Z = 4) atom has 4 electrons. The molecule Be₂ would have 8 electrons. Four electrons form one Be atom and 4 electrons from other Be atom. The molecular orbital configuration of Be₂ molecule is s 1s² < s *1s² < s 2s² < s *2s²

     

 Note: -

  v Bond energy α Bond order, therefore the increasing order of   bond energy is Alkane < Alkene < Alkyne 

v If N_b is greater than N_a then B.O. will be positive, hence the molecule is stable. 

v If N_b is less than N_a then B.O will be negative, hence the molecule is unstable or does not exist. 

v If N_b = N_a then B.O. will be 0, hence the molecule is unstable or does not exist. 

v Isoelectronic species (having same number of electrons) have same bond order.