Which one of the following is the most stable diatomic species?

Concept:

Bond order:

• It is the number of bonds between two atoms.
• The bonding electrons being stable contributes to increasing the bond order.
• The antibonding electrons being unstable contributes to decreasing the bond order.

Bond order is given by

 \(\frac{no. \:of\:bonding \:- \:no. \:of \:anti\:bonding\:electrons}{2}\)

• For filling up of electrons, we need to follow the series: σ 1sσ1s*σ2sσ 2s*σ 2pzπ 2py = π 2px π 2py* = π 2px*σ 2pz*.
• For molecules till nitrogen, there is a slight change in the M.O energy order: 
  • σ 1sσ1s*σ2sσ 2sπ 2py = π 2px*σ 2pz π 2py = *π 2px*σ 2pz*

Bond order and stability:

• Bond order is the number of bonds between two atoms.
• The bonding electrons being stable contributes to increasing the bond order.
• The antibonding electrons being unstable contributes to decreasing the bond order.
• The greater the number of bonds, the higher is the stability.
• The bonding electrons are more stable and bonding M.O is filled first.
• The antibonding M.O is less stable and filled after bonding M.O is filled.
• Bond order is given by

 \(\frac{no. \:of\:bonding \:- \:no. \:of \:anti\:bonding\:electrons}{2}\)

Explanation:

N2 molecule, the electronic configuration will be:

σ 1s2σ1s2*σ2s2σ 2s2π 2py 2= π 2px*²σ 2pz2 π 2py = *π 2px*σ 2pz*.

\(Bond\;order = \frac{{10 - 6}}{2} = 3\)

O2+ molecule, the electronic configuration is:

σ 1s2σ1s*2σ2s2σ 2s*2σ 2pz2π 2py2π 2px²π 2py*1.

\(Bond\;order = \frac{{10 -5 }}{2} = 2.5\)

O2 the electronic configuration is as follows:

​ σ 1s2σ1s*2σ2s2σ 2s*2σ 2pz2π 2py2π 2px2 π 2py*2.

• There are 10 bonding and 6 antibonding electrons.

So, the bond order is:

\(\frac{no. \:of\:bonding \:- \:no. \:of \:anti\:bonding\:electrons}{2}\)

\({1\over 2}{(10-6)}=2\)

O2^- molecule, the electronic configuration is:

 σ 1s2σ1s*2σ2s2σ 2s*2σ 2pz2π 2py2π 2px2 π 2py*2π 2px*¹.

\(Bond\;order = \frac{{10 -7 }}{2} = 1.5\)

Bond order is directly proportional to the stability of molecules.

Hence the most stable molecule is N₂ because it has the highest bond order 3.

Important Points

• For molecules till nitrogen, there is slight change in the M.O energy order: 
• σ 1sσ1s*σ2sσ 2sπ 2py = π 2px*σ 2pz π 2py = *π 2px*σ 2pz* π 2py
• This is due to the difference in mixing of orbitals.