Restless Molecule

Ammonium nitrate, whose blast wrecked Texas City last week (see NATIONAL AFFAIRS) is all too easy to make. The recipe: add ammonia to dilute nitric acid.

Chemically, ammonium nitrate is a salt, a combination of a base and an acid. But it is far from peaceful, as most other salts are. Instead of having a metal (e.g., sodium or iron) as the basic part of its molecule, it has an ammonium "radical" (one nitrogen atom and four hydrogen atoms) masquerading as a metal. Its acid part is also a radical: one nitrogen and three oxygen atoms (see chart).

Ammonium nitrate's outwardly peaceful molecule perpetually strains with suppressed desires. The oxygen and hydrogen atoms are not combined with each other, as their natures prompt them to be. They fret in frustrated juxtaposition, kept from an explosive embrace by a frangible barrier of chemical propriety.

Under ordinary conditions, the straining atoms contain themselves. But a sufficient disturbance, such as heat or shock in the presence of certain impurities, shatters the barrier. Every oxygen atom grabs two hydrogen atoms. Every pair of nitrogen atoms, deserted, grabs a single oxygen. In consummating these unions, the atoms generate enormous heat--and the salt flashes into gases: superheated steam (H2O) and nitrous oxide (N2O).

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