Question

What is meant by the term bond order ? Calculate the bond order of $N_2$, $O_2$ , $O_2^{+}$ and ${O_2}^{-}$.

Answer 1

The number of covalent bonds present in covalent molecule is called bond order. It is one half the difference between the number of electrons in the bonding and antibonding molecular orbitals.

Bond order $=\frac{\text{number of electrons in bonding Molecular orbitals}}{\text{number of electrons in antibonding Molecular orbitals}}$

(i) For $N_2$ molecule,

The electronic configuration is $KK\left(\sigma 2s{)}^2\right({\sigma }^{\ast }2s{)}^2\left(\pi 2p_x{)}^2\right(\pi 2p_y{)}^2(\sigma 2p_z{)}^2$.
Its bond order is $\frac{8-2}{2}=3$.

(ii)The electronic configuration of $O_2$ molecule is $KK\left(\sigma 2s{)}^2\right({\sigma }^{\ast }2s{)}^2\left(\sigma 2p_z{)}^2\right(\pi 2p_x{)}^2\left(\pi 2p_y{)}^2\right({\pi }^{\ast }2p_x{)}^1({\pi }^{\ast }2p_y{)}^1$.
Its bond order is $\frac{8-4}{2}=2$.

(iii)The electronic configuration of $O_2^{+}$ ion is $KK\left(\sigma 2s{)}^2\right({\sigma }^{\ast }2s{)}^2\left(\sigma 2p_z{)}^2\right(\pi 2p_x{)}^2\left(\pi 2p_y{)}^2\right({\pi }^{\ast }2p_x{)}^1$.
Its bond order is $\frac{8-3}{2}=2.5$.

(iv)The electronic configuration of $O_2^{-}$ molecule is $KK\left(\sigma 2s{)}^2\right({\sigma }^{\ast }2s{)}^2\left(\sigma 2p_z{)}^2\right(\pi 2p_x{)}^2\left(\pi 2p_y{)}^2\right({\pi }^{\ast }2p_x{)}^2({\pi }^{\ast }2p_y{)}^1$.
Its bond order is $\frac{8-5}{2}=2.5$.