Question

Identify a molecule which does not exist.

• A. $H{e}_2$
• B. $L{i}_2$
• C. $C_2$
• D. $O_2$

Answer 1

Answer: (A)

Species in which bond order is zero do not exist.
(1) $O_2=(KK)(\sigma 2s^2)({\sigma }^{\ast }2s^2)(\sigma 2p_z^2)$
$(\pi 2p_x^2=\pi 2p_y^2)({\pi }^{\ast }2p_x^1={\pi }^{\ast }2p_y^1)$
Bond Order $=\frac{10-6}{2}=2$
(2) $H{e}_2=(\sigma 1s^2)({\sigma }^{\ast }1s^2)$
Bond Order $=0$
(3) $L{i}_2=(\sigma 1s^2)({\sigma }^{\ast }1s^2)({\sigma }^{\ast }2s^2)$
Bond Order $=1$
(4) $C_2=(\sigma 1s^2)({\sigma }^{\ast }1s^2)(\sigma 2s^2)({\sigma }^{\ast }2s^2)$
$(\pi 2p_x^2=\pi 2p_y^2)$
Bond Order $=2$
As, bond order of $H{e}_2$ is zero it does not exist.