${\text { The electronic configuration of } N O \text { is }}$
${\sigma_{1 s}^{2} \sigma_{1 s}^{* 2} \sigma_{2 s}^{2} \sigma_{2 s}^{o 2} \sigma_{2 p r s}^{2} \pi_{2 p r}^{2}}$
$=\pi_{2 p_{y}}^{2} \pi_{2 p_{z}}^{* 1}$
$Pb \left( NO _{3}\right)_{2} \stackrel{673 K }{\longrightarrow} A + PbO + O _{2}$
$A \stackrel{\text { Dimerise }}{\longrightarrow} B$