Selenium (Se)
Se is a non-metal, chemically related to sulphur and tellurium, and rarely occurs in its elemental state in nature.
Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-gray semi-metal (semiconductor) form that is, in terms of structure, a trigonal polymer chain. It conducts electricity better in the light than in the dark, and is used in photocells (see section Allotropic forms below). Selenium also exists in many non-conductive forms: a black glass-like allotrope, as well as several red crystalline forms built of eight-membered ring molecules, like its lighter cousin sulphur.
| Properties | ||
| Name Phase Density (near r.t.) (gray) Density (near r.t.) (alpha) Density (near r.t.) (vitreous) Liquid density at m.p Melting point Boiling point Critical point Heat of fusion (gray) Heat of vaporisation molar heat capacity | Se Solid 4.81 g.cm-3 4.39 g.cm-3 4.28 g.cm-3 3.99 g.cm-3 494 K, 221 °C, 430 °F 958 K, 685 °C, 1265 °F 1766 K, 27.2 MPa 6.69 kL.mol-1 95.48 kJ.mol-1 25.363 J.mol-1.K-1 | |
Where it comes from
Selenium is found in small quantities in sulphide ores such as pyrite, partially replacing the sulphur in the ore matrix. Selenide or selenate mineral compounds are also known, but are rare.
The chief commercial uses for selenium today are in glassmaking and in chemicals and pigments. Uses in electronics, once important, have been supplanted by silicon semiconductor devices.
Impact on environment and people
Symptoms of selenosis include a garlic odour on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage. Extreme cases of selenosis can result in cirrhosis of the liver, pulmonary oedema, and death.[28] Elemental selenium and most metallic selenides have relatively low toxicities because of their low bioavailability.