Question

According to MOT, which of the following species has only $\pi$ electrons involved in bonding?

• A. $C_2$
• B. $N_2$
• C. $O_2$
• D. $B_2$

Answer 1

Answer: (A), (D)

The correct options are
A $C_2$
D $B_2$

$\Rightarrow N_2\to {\sigma }_{1s}^2{\sigma }_{1s}^{\ast 2}{\sigma }_{2s}^2{\sigma }_{2s}^{\ast 2}{\sigma }_{2p_z}^2{\pi }_{2p_y}^2{\pi }_{2p_x}^2$

$\Rightarrow O_2\to {\sigma }_{1s}^2{\sigma }_{1s}^{\ast 2}{\sigma }_{2s}^2{\sigma }_{2s}^{\ast 2}{\sigma }_{2p_z}^2{\pi }_{2p_y}^2{\pi }_{2p_x}^2{\pi }_{2p_y}^{\ast 1}{\pi }_{2p_x}^{\ast 1}$

$\Rightarrow C_2\to {\sigma }_{1s}^2{\sigma }_{1s}^{\ast 2}{\sigma }_{2s}^2{\sigma }_{2s}^{\ast 2}{\pi }_{2p_y}^2{\pi }_{2p_x}^2$

$\Rightarrow B_2\to {\sigma }_{1s}^2{\sigma }_{1s}^{\ast 2}{\sigma }_{2s}^2{\sigma }_{2s}^{\ast 2}{\pi }_{2p_y}^1{\pi }_{2p_x}^1$

From the electronic configuration,
we can see that $B_2$ and $C_2$ have $\pi$ molecular orbitals that are involved in bonding.
In other words $B_2$ and $C_2$ has only $\pi$ bonds in the outer subshell, i.e. 2$p$ and no $\sigma$ bond in $2p$.