Question

What is meant by term bond order? Write bond orders for ${\mathrm{N}}_2,{\mathrm{O}}_2$?

Answer 1

Bond order:

• “In the Lewis description of covalent bond, the Bond Order is given by the number of bonds between the two atoms in a molecule.”

Step 1: Formula for calculation of bond order:

• Bond order is half the difference between the number of electrons in the bonding orbital and the number of electrons in non-bonding orbitals.
• $\mathrm{Bond}\mathrm{order}=\frac{1}{2}\left[{\mathrm{N}}_{\mathrm{b}}-{\mathrm{N}}_{\mathrm{a}}\right]$
• Where ${\mathrm{N}}_{\mathrm{b}}$ represents the number of electrons in the bonding orbital while ${\mathrm{N}}_{\mathrm{a}}$is the number of electrons in the antibonding orbitals.

Step 2: Calculation of bond order for ${\mathrm{N}}_2$ molecule:

• Electronic configuration of ${\mathrm{N}}_2$ ($14$ electrons) = $\mathrm{KK}\text{'}{\left(\mathrm{\sigma }2\mathrm{s}\right)}^2{(\mathrm{\sigma }*2\mathrm{s})}^2{\left(\mathrm{\pi }2\mathrm{px}\right)}^2{\left(\mathrm{\pi }2\mathrm{py}\right)}^2{\left(\mathrm{\sigma }2\mathrm{pz}\right)}^2$
• The molecular orbital diagram of ${\mathrm{N}}_2$ is given below:

$$\begin{gathered} \mathrm{Bond}\mathrm{order}=\frac{1}{2}\left[8-2\right] \\ \mathrm{Bond}\mathrm{order}=\frac{1}{2}\times 6=3 \end{gathered}$$

Step 3: Calculation of bond order for ${\mathrm{O}}_2$ molecule:

• Electronic configuration of ${\mathrm{O}}_2$ ($16$ electrons) =$KK\text{'}{\left[\mathrm{\sigma }\right(2\mathrm{s}\left)\right]}^2{\left[{\mathrm{\sigma }}^{\ast }\right(2\mathrm{s}\left)\right]}^2{\left[\mathrm{\sigma }\right(1\mathrm{pz}\left)\right]}^2{\left[\mathrm{\pi }\right(2\mathrm{px}\left)\right]}^2{\left[\mathrm{\pi }\right(2\mathrm{py}\left)\right]}^2{\left[{\mathrm{\pi }}^{\ast }\right(2\mathrm{px}\left)\right]}^1{\left[{\mathrm{\pi }}^{\ast }\right(2\mathrm{py}\left)\right]}^1$
• The molecular orbital diagram of ${\mathrm{O}}_2$ is given below:

$$\begin{gathered} \mathrm{Bond}\mathrm{order}=\frac{1}{2}\left[8-4\right] \\ \mathrm{Bond}\mathrm{order}=\frac{1}{2}\times 4=2 \end{gathered}$$