Pnictogen hydride
Pnictogen hydrides or hydrogen pnictides are binary compounds of hydrogen with pnictogen atoms (elements of group 15: nitrogen, phosphorus, arsenic, antimony, and bismuth) covalently bonded to hydrogen.
The simplest series has the chemical formula XH3 (less commonly H3X), with X representing any of the pnictogens. They take on the pyramidal structure (as opposed to the trigonal planar arrangement of the group 13 hydrides) and therefore are polar. These pnictogen trihydrides are generally increasingly unstable and poisonous with heavier elements.
Similar to that of the simple hydrogen halides and chalcogenides, the pnictogen hydrides are water-soluble. Interestingly, unlike other hydrides such as hydrogen sulfide and hydrogen fluoride, with acidic aqueous solutions, ammonia dissolves in water to make ammonium hydroxide, which is basic (by forming a hydroxide ion as opposed to hydronium). Phosphine is also water-soluble. Arsine and stibine are even less so, and it is not known if bismuthine is.
Some properties of the pnictogen trihydrides follow:
Interestingly, the gases have no smell in pure form; instead gaining these when in contact with air. Ammonia has the infamous, intense odour resembling urine and/or fish, commonly the result of the decomposition of urea. Phosphine smells like fish or garlic, and stibine like rotten eggs, similar to hydrogen sulfide and selenide.
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