Molecular orbital theory and stability of helium molecule

Q.Why helium(He2) molecule does not exist ?

The existence of He2 molecule can be explain by molecular orbital theory.According to    molecular orbital theory the bond order of helium molecule is zero. That  is no bond is formed between two helium molecule. so it does not exist.

 M.O diagram of helium molecule is shown below.

Bonding electron ( Nb ) =2 , Antibonding electron (Na )=2
so, Bond order =0

Why does the bond dissociation energy of N₂ is greater than N₂ ⁺ ion ?

The bond dissociation energy of N₂ is greater than N₂ ⁺ ion . This can be explained on the basis of molecular orbital theory . According to molecular orbital theory , the electronic configuration of N₂ molecule are shown below,

From the above electronic configuration , it has been found that the number of bonding electron( N_b) is 10 and the number of anti bonding electron( N_a) is 4.

On the other, the molecular orbital electronic configuration  of N₂ ⁺ ion is ,

 From the above electronic configuration , it has been found that the number of bonding electron( N_b ) is 9 and the number of anti bonding electron( N_a) is 4.

Now, the bond dissociation energy of molecule depends on their bond order. Since the bond order of N₂ (3) is higher than N₂ ⁺(2.5) ion , consequently, the energy required to break the N₂ bond is greater than N₂ ⁺ ion.

For this reason, the bond dissociation energy of N₂ is greater than N₂ ⁺ ion.

Molecular orbital theory (M.O) :