Question

If Hund's rule is not applicable, then how many unpaired $e^{-}$ are present in $B_2$ molecule ?

Answer 1

If Hund's rule is not applicable, then zero many unpaired $e^{-}$ are present in $B_2$ molecule .
The molecular orbital configuration of $B_2$ is $KK\sigma 2s^2\sigma \ast 2s^2,\pi 2p{y}^1\pi 2p{y}^1$
However if the Hund's rule is not followed then the molecular orbital configuration will be $KK\sigma 2s^2\sigma \ast 2s^2,\pi 2p{y}^2\pi 2p{y}^0$. hence, all the electrons will b paired.