Question

Explain why He2 does not exist under ordinary condition? Calculate bond order of He2+.?
Please give me Full solution

Answer 1

Dear student

1.According to Molecular orbital theory:
Helium Molecule [He₂]:
The electronic configuration of helium atom is 1s. Each helium atom contains 2 electrons. Therefore, in He₂ molecule there would be 4 electrons which will be accommodated in σ 1s and σˣ 1s molecular orbits leading to electronic configuration:
He2:(σ1s)²(σ ˣ 1s)²
Bond order of He₂ is 1/2(Nb-Na) where Na=electron in anti bonding orbital , Nb=electron in bonding orbital]
=1/2(2-2)=0
He2 molecule is therefore unstable and does not exist.
Bond Order of He₂⁺ : 
As He atom contains 2 electrons He₂⁺ ion will contain 3 electrons.
2 electrons will enter bonding σ(1s) molecular orbital and third electron will enter anti bonding σˣ(1s) molecular orbital.
Thus electronic configuration of He₂⁺ will be:
[σ(1s)² (σˣ1s)¹]
here Nb=[electron in bonding orbital]=2
Na=[electron in anti bonding orbital]=1
Bond order=1/2[Nb-Na]
=1/2[2-1]=1/2
Hope it is clear.