Question

Identify a molecule which does not exist.

• A. $He_2$
• B. $Li_2$
• C. $C_2$
• D. $O_2$

Answer 1

Answer: (A)

$He_2$

Answer 2

Species in which bond order is zero do not exist.
(1) $O_2 = (KK)(\sigma 2s^2)(\sigma^* 2s^2)(\sigma 2p_z^2)$
$(\pi 2p_x^2 = \pi 2p_y^2)( \pi^* 2p_x^1 = \pi^* 2p_y^1)$
Bond Order $= \frac{10 - 6}{2} = 2$
(2) $He_2 = (\sigma 1s^2)(\sigma^* 1s^2)$
Bond Order $= 0$
(3) $Li_2 = (\sigma1s^2)(\sigma^* 1s^2)(\sigma^* 2s^2)$
Bond Order $= 1$
(4) $C_2 = (\sigma 1s^2)(\sigma^* 1s^2)(\sigma 2s^2)(\sigma^* 2s^2)$
$(\pi 2p_x^2 = \pi 2p_y^2)$
Bond Order $= 2$
As, bond order of $He_2$ is zero it does not exist.