Selenium

About Selenium

Selenium sits below sulfur in group 16, and it inherits a lot of sulfur's chemistry while adding a few twists of its own. It exists in several allotropes — red amorphous, black vitreous, and the technologically interesting gray trigonal form built from helical chains of Se atoms. Gray selenium is a photoconductor: its conductivity jumps by orders of magnitude under illumination, which is why Chester Carlson built the first xerographic plates from it in 1938 and why early photographic light meters worked. Biologically it sits at one of the narrowest beneficial-to-toxic windows of any nutrient. Selenocysteine, encoded by a recoded UGA codon, is the active-site residue in glutathione peroxidases and thioredoxin reductases — enzymes that quench peroxides and keep oxidative stress in check. Soils in parts of the US Great Plains and northern China carry enough selenium to cause livestock 'alkali disease', while soils in Keshan County, China, were so depleted that they triggered a deficiency-linked cardiomyopathy. The redox chemistry runs from Se(VI) selenate down to Se(-II) selenide, and the smell of dimethyl selenide is famously, persistently bad.