Question

Question 09

Which molecule/ion out of the following does not contain upaired electons?

(a) $N_2^{+}$

(b) $O_2$

(c) $O_2^{2-}$

(d) $B_2$

Answer 1

Answer: (c) $O_2^{2-}$

As per the molecular orbital theory, to check the unpaired electrons.

The electronic configuration of the given molecules are

$N_2^{+}=\sigma 1s^2,{\sigma }^{\ast }1s^2,\sigma 2s^2,{\sigma }^{\ast }2s^2,\pi 2p_x^2=\pi p_y^2,\sigma 2p_z^1$

It has one upaired electron.

$O_2=\sigma 1s^2,{\sigma }^{\ast }1s^2,\sigma 2s^2,{\sigma }^{\ast }2s^2,\sigma 2p_z^2,\pi 2p_x^2=\pi 2p_y^2,\pi \ast 2p_x^1=\pi \ast 2p_y^1$

$O_2$ has two unpaired electrons.

$O_2^{-2}=\sigma 1s^2,{\sigma }^{\ast }1s^2,\sigma 2s^2,{\sigma }^{\ast }2s^2,\sigma 2p_z^2,\pi 2p_x^2=\pi 2p_y^2,\pi \ast 2p_x^2=\pi \ast 2p_y^2$

Thus, $O_2^{-2}$ has no unpaired electrons.

$B_2=\sigma 1s^2,{\sigma }^{\ast }1s^2,\sigma 2s^2,{\sigma }^{\ast }2s^2,\pi 2p_x^1\approx {\pi }_2{p}_y^1$

Thus, $B_2$ has two unpaired electrons.