Question

If Hund's rule does not hold good, then which of the following pairs is diamagnetic?

• A. $B_2,O_2^{+}$
• B. $C_2,O_2$
• C. $O_2^{-},N_2$
• D. $N_2,NO$

Answer 1

Answer: (B)

$C_2(12):$ - Violating Hund's rule

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

$={\pi }^{\ast }2p_y,{\sigma }^{\ast }2p_z$

$C_2$ is diamagnetic.

For $O_2(16e.)$ violating Hund's rule

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

$={\pi }^{\ast }2p_y,{\sigma }^{\ast }2p_z$

$O_2$, is also diamagnetic