Ammonia

Ammonia is a compound with the formula NH3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers. Ammonia, either directly or indirectly, also is a building block for the synthesis of many pharmaceuticals. Although in wide use, ammonia is both caustic and hazardous. It is infamous for being mixed with the household cleaning product, bleach, and forming an extremely toxic gas through reaction.

Background
The ammonia molecule has a trigonal pyramidal shape, as predicted by VSEPR theory. The nitrogen atom in the molecule has a lone electron pair, and ammonia acts as a base, a proton acceptor. This shape gives the molecule a dipole moment and makes it polar so that ammonia readily dissolves in water. The degree to which ammonia forms the ammonium ion increases upon lowering the pH of the solution— at "physiological" pH (~7), about 99% of the ammonia molecules are protonated. Temperature and salinity also affect the proportion of NH4+. NH4+ has the shape of a regular tetrahedron.

Ammonia is found in small quantities in the atmosphere, being produced from the putrefaction of nitrogenous animal and vegetable matter. Ammonia and ammonium salts are also found in small quantities in rainwater. Ammonium salts also are found distributed through all fertile soil and in seawater. Substances containing ammonia, or those that are similar to it, are called ammoniacal.

Clouds of white ammonia are present in the atmosphere of Threshold, along with Ammonium Hydrosulfide, Phosphorus compounds, ice crystals, and other gases.

List of appearances

 * Halo: First Strike