Among the following molecules/ions C22-, N22-, O22-, O2, which one is diamagnetic and has the shortest bond length?

🧠 The species with the highest bond order will have the shortest bond, and it must be diamagnetic (all electrons paired).

Bond orders and magnetic nature:

| Species | Total e⁻ | Bond Order | Unpaired e⁻ | Magnetic | |---------|-----------|-----------|------------|---------| | $\mathrm{C_2^{2-}}$ | 14 | $(10-4)/2 = 3$ | 0 | Diamagnetic | | $\mathrm{N_2^{2-}}$ | 16 | $(10-6)/2 = 2$ | 2 | Paramagnetic | | $\mathrm{O_2^{2-}}$ | 18 | $(8-8)/2 = 1$ | 0 | Diamagnetic | | $\mathrm{O_2}$ | 16 | $(10-6)/2 = 2$ | 2 | Paramagnetic |

🗺️ MO config of $\mathrm{C_2^{2-}}$ (14e): \sigma_{1s}^2\sigma^*_{1s}^2\sigma_{2s}^2\sigma^*_{2s}^2\pi_{2p}^4\sigma_{2p}^2

BO = $(10-4)/2 = 3$ — triple bond → shortest bond length among these four.

⚠️ Two species are diamagnetic: $\mathrm{C_2^{2-}}$ (BO = 3) and $\mathrm{O_2^{2-}}$ (BO = 1). Only $\mathrm{C_2^{2-}}$ satisfies both criteria.

$\boxed{\text{Answer: } \mathrm{C_2^{2-}}}$