Identify the pseudohalogen from the following ions:

The correct answer is Option (2) → CN⁻

A pseudohalide ion is a polyatomic, singly charged anion that behaves like a halide ion \( \ce{X^{-}} \): it forms an insoluble silver salt, combines with \( \ce{H^{+}} \) to give an acid HX, acts as a ligand, and can be oxidised to a neutral dimer, the pseudohalogen, which behaves like \( \ce{X2} \). Common examples are \( \ce{CN^{-}} \), \( \ce{SCN^{-}} \), \( \ce{OCN^{-}} \) and \( \ce{N3^{-}} \).

The cyanide ion fits all these points: it gives AgCN (insoluble, like AgCl), HCN (an acid, like HCl), complexes such as \( \ce{[Ag(CN)2]^{-}} \), and on oxidation the pseudohalogen cyanogen, (CN)₂, which is analogous to \( \ce{Cl2} \):

    \( \displaystyle \ce{2Cu^2+ + 4CN^- -> 2CuCN + (CN)2} \)

So among the given ions, CN⁻ is the pseudohalide (pseudohalogen) ion.

Why the other options are incorrect

• Option (1) BrO₃⁻ — Incorrect. The bromate ion is an oxoanion of a halogen (Br in +5 state), a strong oxidising agent; it does not behave like a halide ion.

• Option (3) NO₂⁻ — Incorrect. Nitrite is an oxoanion of nitrogen; it does not form a dimer like \( \ce{X2} \) or an insoluble silver salt in the halide pattern.

• Option (4) Cl⁻ — Incorrect. Chloride is a true halide ion, not a pseudohalide.

Note: The question is from beyond the NCERT textbook, and it is also out of the CUET 2026 syllabus.