IIT-JEE 2009 · Paper 1, Chemistry question 19

MO configurations. For $\ce{B2}$, $\ce{C2}$, $\ce{N2}$ s-p mixing puts $\pi_{2p}$ below $\sigma_{2p}$; for O it does not.

• A, $\ce{B2}$ (10 e): $\pi_{2p}^1\pi_{2p}^1$, two unpaired electrons, bond order 1; s-p mixing; it can lose or gain electrons. → P, Q, R, T.
• B, $\ce{N2}$ (14 e): bond order 3, diamagnetic; s-p mixing; it can be oxidised ($\ce{N2+}$, oxides) or reduced ($\ce{NH3}$, nitrides). → Q, R, S, T.
• C, $\ce{O2-}$ (17 e): one unpaired electron, bond order 1.5; can be oxidised to $\ce{O2}$ or reduced to $\ce{O2^{2-}}$. → P, Q, R.
• D, $\ce{O2}$ (16 e): two unpaired electrons, bond order 2; can be oxidised ($\ce{O2+}$) or reduced. → P, Q, R, S.

Answer: A → P, Q, R, T; B → Q, R, S, T; C → P, Q, R; D → P, Q, R, S