Hydrogen chalcogenide
Hydrogen chalcogenides (also chalcogen hydrides or hydrogen chalcides) are binary compounds of hydrogen with chalcogen atoms (elements of group 16: oxygen, sulfur, selenium, tellurium, and polonium). Water, one of the earliest chemical compounds in this series, contains one oxygen atom and two hydrogen atoms, and is the most common compound on the Earth's surface.
The most important series, including water, has the chemical formula H2X, with X representing any chalcogen. They are therefore triatomic. They take on a bent structure and as such are polar molecules. Water is an essential compound to life on Earth today, covering 70.9% of the planet's surface. The other hydrogen chalcogenides are usually extremely toxic, and have strong unpleasant scents usually resembling rotting eggs or vegetables. Hydrogen sulfide is a common product of decomposition in oxygen-poor environments and as such is one chemical responsible for the smell of flatulence. It is also a volcanic gas. Despite its toxicity, the human body intentionally produces it in small enough doses for use as a signaling molecule.
Water can dissolve the other hydrogen chalcogenides (at least those up to hydrogen telluride), forming acidic solutions known as hydrochalcogenic acids. Although these are weaker acids than the hydrohalic acids, they follow a similar trend of acid strength increasing with heavier chalcogens, and also form in a similar way (turning the water into a hydronium ion H3O+ and the solute into a XH− ion). It is unknown if polonium hydride forms an acidic solution in water like its lighter homologues, or if it behaves more like a metal hydride (see also hydrogen astatide).
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