ChalcogenThe chalcogens are the name for the periodic table group 16 (old-style: VIB or VIA) in the Periodic Table. It consists of the elements Oxygen (O), Sulfur (S), Selenium (Se), Tellurium (Te), and the radioactive Polonium (Po).
Their compounds, particularly the sulfides, selenides and tellurides are collectively known as chalcogenides. A term like 'chalcides' might have been a more logical derivative from chalcogen, like halide is from halogen, but the scientific language is, like any language, not always logical.
Oxygen and sulfur are nonmetals, polonium is a true metal, and selenium and tellurium are metalloid semiconductors (i.e., their electrical properties are between those of a metal and an insulator). Nevertheless, tellurium, as well as selenium, is often referred to as a metal when in elemental form.
The oxidation number of the chalcogen is generally -2 in a chalcogenide but other values (e.g. -1 in pyrite) can be attained.
The highest oxidation number +6 is found in sulfates, selenates and tellurates, e.g. in Na2SeO4 sodium selenate
See also: Periodic table of elements