Hydrogen gas is diatomic where as inert gases are monoatomic - explain on the basis of MO theory.

Explanation

Hydrogen gas is diatomic where as inert gases are monoatomic because noble gases are having its valence shell filled with octet electrons.
So there is no necessity for the noble gases to involve in the formation of covalent bonds.
This could be further explained by MO diagram of `H_(2)" and " He_(2)`.
Molecular orbital diagram of hydrogen molecule `(H_(2))`:
Electronic configuration of H atom `1 s^(1)`
Electronic configuration of `H_(2)" molecule " sigma_(1s)^(2)`
Bond order ` = (N_(b) - N_(a))/2 = (2 -0)/2 = 1`
Molecule has no unpaired electrons hence it is diamagnetic.

The electronic configuration of `He (Z = 2)` in the ground state is `1 s^(2)`.
As each helium atom contains two electrons, there will be four electrons in `He_(2)` molecule. Keeping in view the Aufbau principle and Pauli's exclusion principle its electronic configuration would be as follows :
`He_(2) : ( sigma_(1s))^(2) ( sigma_(1s)^(**))^(2)`
` N_(b) = 2 ` and
`N_(a) = 2 :. " Bond order " = (N_(b) - N_(a))/2 = (2 - 2)/2`
As the bond order of `He_(2)` come out to be zero, this molecule does not exist.