Question

How many unpaired electrons are present in the highest occupied molecular orbital of $O_2$ molecule?

Answer 1

$O_2$ has 16 electrons
$(\sigma 1s{)}^2,({\sigma }^{\ast }1s{)}^2,(\sigma 2s{)}^2,({\sigma }^{\ast }2s{)}^2,(2p_z{)}^2,(\pi {)}^4,({\pi }^{\ast }{)}^2$
In HOMO, it has two electrons.