JEE Advanced 2014 · Paper 2, Chemistry question 30

Without 2s-2p mixing the order is $\sigma2s<\sigma^*2s<\sigma2p_z<\pi2p_x=\pi2p_y<\pi^*2p<\sigma^*2p_z$.

Valence electrons:

• $\ce{Be2}$ (4): $\sigma2s^2\sigma^*2s^2$. Diamagnetic.
• $\ce{B2}$ (6): $\ldots\sigma2p_z^2$. Diamagnetic.
• $\ce{C2}$ (8): $\ldots\sigma2p_z^2\,\pi2p_x^1\pi2p_y^1$. Two unpaired electrons: paramagnetic.
• $\ce{N2}$ (10): $\ldots\sigma2p_z^2\pi2p^4$. Diamagnetic.

(C)