Q.9 Among the following pairs, the paramagnetic and diamagnetic species, respectively, are
(A) CO and O2− (B) NO and CO (C) O22−and CO (D) NO+and O2−
CO is diamagnetic while superoxide ion O₂⁻ is paramagnetic, making option (A) the correct choice among the given pairs. This determination relies on molecular orbital theory, where unpaired electrons cause paramagnetism and all paired electrons result in diamagnetism.
Core Concepts
Paramagnetic species have unpaired electrons and are attracted to magnetic fields, while diamagnetic species have all paired electrons and are weakly repelled. Molecular orbital (MO) diagrams predict this by filling orbitals according to the Aufbau principle for diatomic molecules. Total valence electrons dictate pairing: even numbers often pair fully in stable molecules like CO (14 electrons), while odd numbers or extra electrons create unpaired spins as in NO (15 electrons).
Option Analysis
Each option pairs one paramagnetic and one diamagnetic species candidate. Here’s the breakdown using MO configurations (focusing on 2s/2p orbitals, simplified for homonuclear/heteronuclear diatomics).
| Option | First Species | Electrons | MO Key Features | Nature | Second Species | Electrons | MO Key Features | Nature |
|---|---|---|---|---|---|---|---|---|
| (A) | CO | 14 | All paired: (σ2s)²(σ*2s)²(π2p)⁴(σ2p)² | Diamagnetic | O₂⁻ | 17 | Two unpaired in π*2p (like O₂ with extra e⁻) | Paramagnetic |
| (B) | NO | 15 | One unpaired in π*2p (odd electron count) | Paramagnetic | CO | 14 | All paired | Diamagnetic |
| (C) | O₂²⁻ | 18 | All paired: π*2p fully occupied (peroxide ion) | Diamagnetic | CO | 14 | All paired | Diamagnetic |
| (D) | NO⁺ | 14 | Removes unpaired e⁻ from NO; all paired like N₂/CO | Diamagnetic | O₂⁻ | 17 | Two unpaired | Paramagnetic |
Only (A) matches one of each; (C) has both diamagnetic, others reverse the pairing.
Why Option A Wins
CO’s MO diagram mirrors N₂: bonding orbitals fill completely with no unpaired electrons, yielding bond order 3 and diamagnetism confirmed experimentally. O₂⁻ (superoxide) builds on O₂’s two unpaired π* electrons but adds one more, still leaving unpaired spins and paramagnetism vital in biochemistry (e.g., oxidative stress). This pair uniquely fits the question’s criteria.
