In which of the following ionization processes, the bond order has increased and the magnetic behaviour has changed?

Solution:

The order of increasing energy of molecular orbitals is 

$\left(\sigma 2\mathrm{s}\right)\left({\sigma }^{\ast }2\mathrm{s}\right)\left(\pi 2{\mathrm{p}}_x\right)\left(\pi 2{\mathrm{p}}_y\right)\left(\sigma 2{\mathrm{p}}_z\right)\left({\pi }^{\ast }2{\mathrm{p}}_x\right)\left({\pi }^{\ast }2{\mathrm{p}}_y\right)\left({\sigma }^{\ast }2{\mathrm{p}}_z\right)$

The valence electrons in ${\mathrm{C}}_2,\mathrm{NO},{\mathrm{O}}_2$ and ${\mathrm{N}}_2$ are 8, 11, 12 and 10, respectively. The given criteria is satisfied only by $\mathrm{NO}$. Its configuration is

$\left(\sigma 2\mathrm{s}{)}^2\right(\sigma \ast 2\mathrm{s}{)}^2{\left(\pi 2{\mathrm{p}}_x\right)}^2{\left(\pi 2{\mathrm{p}}_y\right)}^2{\left(\sigma 2{\mathrm{p}}_2\right)}^2{\left({\pi }^{\ast }2{\mathrm{p}}_x\right)}^1$

In converting $\mathrm{NO} \mathrm{to} \mathrm{NO}+$, the electron is removed from antibonding molecular orbital and the resultant ion possesses no unpaired of electrons.