Question

Which species contain only $\pi -$bond among the following?

• A. $O^{2-}$
• B. $O_2$
• C. $B_2$
• D. $C_2^{2-}$

Answer 1

The correct option is C $B_2$

$O^{2-}$	$1\sigma \text{bond}$
$O_2$	$1\sigma \text{and}1\pi$
$B_2$	$1\pi \text{bond only}$
$C_2^{2-}$	$1\sigma \text{and}2\pi \text{bonds}$

Molecular electronic configuration of $O_2=\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\sigma 2p_x\pi 2p_y^2=\pi 2p_z^2,{\pi }^{\ast }2p_y^1={\pi }^{\ast }2p_z^1$
Molecular electronic configuration of $B_2=\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\pi 2p_y^1=\pi 2p_z^1$
Molecular electronic configuration of $C_2^{2-}=\sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\pi 2p_y^2=\pi 2p_z^2\sigma 2p_x^2$

$B_2$ possess only $\pi$ bond, rest all molecules possess sigma bond or sigma bond and pi bond.