Question

Among the following the molecule expected to be stabilized by anion formation is ${\mathrm{C}}_2,{\mathrm{O}}_2,\mathrm{NO},{\mathrm{F}}_2$?

Answer 1

The stabilization by anion formation can be seen by calculating the bond order which tells whether the incoming electron will enter the bonding or the anti-bonding orbital.

1.${\mathrm{C}}_2$

• The number of valence electrons in ${\mathrm{C}}_2$are $12$.
• The electronic configuration ${\mathrm{C}}_2$:${\left(\mathrm{\sigma }1\mathrm{s}\right)}^2{(\mathrm{\sigma }*1\mathrm{s})}^2{\left(\mathrm{\sigma }2\mathrm{s}\right)}^2{(\mathrm{\sigma }*2\mathrm{s})}^2(\mathrm{\pi }2{\mathrm{px}}^2=\mathrm{\pi }2{\mathrm{py}}^2)$
• $$\begin{gathered} \mathrm{Bond}\mathrm{order}=\frac{(\mathrm{number}\mathrm{of}\mathrm{electrons}\mathrm{in}\mathrm{bonding}\mathrm{MOT}–\mathrm{number}\mathrm{of}\mathrm{electrons}\mathrm{in}\mathrm{antibonding}\mathrm{MOT})}{2} \\ \mathrm{Bond}\mathrm{order}=\frac{\left(8-4\right)}{2}=\frac{4}{2} \\ \mathrm{Bond}\mathrm{order}=2 \end{gathered}$$
• In ${\mathrm{C}}_2$ the incoming electron will enter the bonding molecular orbital which increases the bond order and stability.

2. ${\mathrm{O}}_2$

• The number of valence electrons in ${\mathrm{O}}_2$are $16$.
• The electronic configuration of ${\mathrm{O}}_2$: ${\left(\mathrm{\sigma }1\mathrm{s}\right)}^2{(\mathrm{\sigma }*1\mathrm{s})}^2{\left(\mathrm{\sigma }2\mathrm{s}\right)}^2{(\mathrm{\sigma }*2\mathrm{s})}^2{\left(\mathrm{\sigma }2\mathrm{pz}\right)}^2(\mathrm{\pi }2{\mathrm{px}}^2=\mathrm{\pi }2{\mathrm{py}}^2)(\mathrm{\pi }*2{\mathrm{px}}^1=\mathrm{\pi }*2{\mathrm{py}}^1)$
• $$\begin{gathered} \mathrm{Bond}\mathrm{order}=\frac{(\mathrm{number}\mathrm{of}\mathrm{electrons}\mathrm{in}\mathrm{bonding}\mathrm{MOT}–\mathrm{number}\mathrm{of}\mathrm{electrons}\mathrm{in}\mathrm{antibonding}\mathrm{MOT})}{2} \\ \mathrm{Bond}\mathrm{order}=\frac{\left(10-6\right)}{2}=\frac{4}{2} \\ \mathrm{Bond}\mathrm{order}=2 \end{gathered}$$
• In ${\mathrm{O}}_2$ the incoming electron will enter the anti-bonding molecular orbital which decreases the bond order and stability

3. $\mathrm{NO}$

• The number of valence electrons in $\mathrm{NO}$ are $15$.
• The electronic configuration of $\mathrm{NO}$: ${\left(\mathrm{\sigma }1\mathrm{s}\right)}^2{(\mathrm{\sigma }*1\mathrm{s})}^2{\left(\mathrm{\sigma }2\mathrm{s}\right)}^2{(\mathrm{\sigma }*2\mathrm{s})}^2{\left(\mathrm{\sigma }2\mathrm{pz}\right)}^2(\mathrm{\pi }2{\mathrm{px}}^2=\mathrm{\pi }2{\mathrm{py}}^2)(\mathrm{\pi }*2{\mathrm{px}}^1)$
• $$\begin{gathered} \mathrm{Bond}\mathrm{order}=\frac{(\mathrm{number}\mathrm{of}\mathrm{electrons}\mathrm{in}\mathrm{bonding}\mathrm{MOT}–\mathrm{number}\mathrm{of}\mathrm{electrons}\mathrm{in}\mathrm{antibonding}\mathrm{MOT})}{2} \\ \mathrm{Bond}\mathrm{order}=\frac{\left(10-5\right)}{2}=\frac{5}{2} \\ \mathrm{Bond}\mathrm{order}=2.5 \end{gathered}$$
• In $\mathrm{NO}$ the incoming electron will enter the anti bonding molecular orbital which decreases the bond order and stability

4. ${\mathrm{F}}_2$

• The number of valence electrons in ${\mathrm{F}}_2$ are $14$.
• The electronic configuration of ${\mathrm{F}}_2$ : ${\left(\mathrm{\sigma }1\mathrm{s}\right)}^2{(\mathrm{\sigma }*1\mathrm{s})}^2{\left(\mathrm{\sigma }2\mathrm{s}\right)}^2{(\mathrm{\sigma }*2\mathrm{s})}^2{\left(\mathrm{\sigma }2\mathrm{pz}\right)}^2(\mathrm{\pi }2{\mathrm{px}}^2=\mathrm{\pi }2{\mathrm{py}}^2)(\mathrm{\pi }*2{\mathrm{px}}^2=\mathrm{\pi }*2{\mathrm{py}}^2)$
• $$\begin{gathered} \mathrm{Bond}\mathrm{order}=\frac{(\mathrm{number}\mathrm{of}\mathrm{electrons}\mathrm{in}\mathrm{bonding}\mathrm{MOT}–\mathrm{number}\mathrm{of}\mathrm{electrons}\mathrm{in}\mathrm{antibonding}\mathrm{MOT})}{2} \\ \mathrm{Bond}\mathrm{order}=\frac{\left(10-8\right)}{2}=\frac{2}{2} \\ \mathrm{Bond}\mathrm{order}=1 \end{gathered}$$
• In ${\mathrm{F}}_2$ the incoming electron will enter the anti-bonding molecular orbital which decreases the bond order and stability.

Thus, In ${\mathrm{C}}_2$ the incoming electron will enter the bonding molecular orbital which increases the bond order and stability while in other molecules it will enter the bonding orbital.