Question

How many unpaired electron(s) is/are present in $O_2$ molecule if Hund's rule is violated ?

Answer 1

If Hund's rule is violeted, then oxygen will contain all paired electrons. According to the Hund's rule of maximum multiplicity, all the orbitals wlll be singly occupied before the pairing starts. Hence, when Hund's rule is violetd, there will be no singly occupied orbital. The electronic configuration of the $O_2$ containing 16 electrons by violating Hund's Rule can be written as:
$1s^2\ast 1s^{2}2s^2\ast 2s^{2}2p{z}^{2}2p{x}^{2}2p{y}^2\ast 2p{x}^2$.