Question

Hydrogen gas is diatomic whereas inert gases are monoatomic – Explain on the basis of MO theory.

Answer 1

The molecular orbital electronic configuration of the hydrogen molecule is (σ_1s²). The molecular orbital energy level diagram of the H₂ molecule is given in

Here, N₂ = 2, N_a = 0

Bond order = ${\displaystyle \frac{{\text{N}}_{\text{b}}-{\text{N}}_{\text{a}}}{2}}$

${\displaystyle =\frac{2-0}{2}}$ = 1

He₂: σ_1s² σ_1s^*2

The molecular orbital energy level diagram of He₂ (hypothetical) is given in

Here, N_b = 2 and N_a = 2

Bond order = ${\displaystyle \frac{{\text{N}}_{\text{b}}-{\text{N}}_{\text{a}}}{2}}$

${\displaystyle \frac{2-2}{2}}$ = 0

As the bond order for He₂ comes out to form between two hydrogen atoms. But as the bond order of helium is zero, there is no bond between helium atoms and hence it is monoatomic.

Result:
As the bond order of the H₂ molecule is one, it is diatomic and a single bond is formed between two hydrogen atoms. But as the bond order of helium is zero, there is no bond between helium atoms and hence it is monoatomic.