Answer to Question #19633 in Inorganic Chemistry for K Jeyakanthan

The nitronium ion, or sometimes the nitryl ion (incorrect because it is not a radical), NO₂⁺, is a generally reactive cation created by the removal of an electron from the paramagnetic nitrogen dioxide molecule, or the protonation of nitric acid.

It is reactive enough to exist in normal conditions, but it is used extensively as an electrophile in the nitration of other substances. The ion is generated in situ for this purpose by mixing concentrated sulfuric acid and concentrated nitric acid according to the equilibrium:

2H₂SO₄ + HNO₃ → 2HSO₄^− + NO₂⁺ + H₃O⁺