Question

If Hund's rule is not applicable, then how many unpaired electrons are present in $NO$ molecule ?

Answer 1

If Hund's rule is not applicable, then $NO$ will contain one unpaired electron out of a total of $15$ electrons($7$ from nitrogen and $8$ from oxygen). According to the Hund's rule of maximum multiplicity, all the orbitals will be singly occupied before the pairing starts. Hence, when Hund's rule is not applicable, there will be only one singly occupied orbital. Thus, out of $15$ electrons of $NO$ molecule, $14$ will be paired and $1$ will be unpaired.