Selenium

Selenium occupies a peculiar position in the periodic table and in human health — it is simultaneously an essential trace nutrient and a potentially toxic element, with the gap between beneficial and harmful doses being surprisingly narrow. Discovered in 1817 when Jöns Jacob Berzelius and Johan Gottlieb Gahn noticed an annoying reddish residue contaminating the sulfuric acid they were producing, the element was initially thought to be tellurium before Berzelius recognized it as something new. He named it after Selene, the Greek goddess of the moon, a nod to tellurium's own name derived from the Earth. Selenium's most striking property is its photoconductivity: it conducts electricity far better when exposed to light than in the dark, a behavior that launched an entire industry.

The photoconducting property of selenium put it at the heart of xerography, the process behind every laser printer and photocopier. A selenium-coated drum, given an electrostatic charge and exposed to light reflected from a document, retains charge only where the image is dark — toner then adheres to those charged regions and transfers to paper. Though modern machines increasingly use organic photoconductors, selenium drums remained standard for decades. Glass manufacturers use selenium compounds to counteract the greenish tint that iron impurities impart, producing optically clear glass, and to produce ruby-red glass for decorative and signal applications. Thin-film solar cells based on copper indium gallium selenide (CIGS) offer a flexible, cost-competitive alternative to silicon photovoltaics. Selenium is also added to stainless steel and copper alloys to improve machinability.

Berzelius and Gahn identified selenium in 1817 while analyzing deposits from the lead chambers of a Swedish sulfuric acid plant in Gripsholm. Berzelius initially suspected the red residue was tellurium, but careful analysis revealed a new element with distinct properties. He announced the discovery in 1818 and named it selene after the moon. For most of the nineteenth century selenium was a laboratory curiosity, prized mainly as a pigment source. The discovery of its photoconductivity in 1873 by Willoughby Smith during cable-testing experiments opened a new era: by the 1880s, scientists including Alexander Graham Bell were experimenting with selenium cells for transmitting sound via light in the photophone. Chester Carlson's invention of xerography in 1938, commercialized by Xerox in the 1950s, made selenium a mass-market industrial material for the first time.

Selenium is a relatively rare element, averaging about 0.05 parts per million in Earth's crust. It is almost never found as a native element; it most commonly occurs as selenide minerals associated with sulfide ore deposits of copper, nickel, and lead. Clausthalite (lead selenide), naumannite (silver selenide), and crookesite (copper selenide) are among the known selenium minerals, but the primary commercial source is the anode slime left over from the electrolytic refining of copper, where selenium concentrates as a byproduct. Selenium content in soils varies dramatically by region — parts of the western United States, Ireland, and China have selenium-rich soils, while other areas are notably deficient, creating corresponding variation in the selenium content of locally grown food.

Selenium dioxide (SeO2) is a key industrial compound used as an oxidizing agent in organic synthesis and as a precursor to other selenium chemicals. Hydrogen selenide (H2Se) is a highly toxic, foul-smelling gas analogous to hydrogen sulfide. Sodium selenite (Na2SeO3) and sodium selenate (Na2SeO4) are the forms used as dietary supplements and in animal feed. Copper indium gallium selenide (CIGS) is a photovoltaic semiconductor with a tunable bandgap ideal for thin-film solar cells. Selenium sulfide (SeS2) is the active ingredient in certain anti-dandruff shampoos, where it slows the rapid skin-cell turnover associated with seborrheic dermatitis. Organoselenium compounds such as selenomethionine are the dominant dietary forms of selenium in plants, and selenocysteine is a genetically encoded amino acid found at the active sites of several important enzymes including glutathione peroxidase.

• Selenium is one of only a handful of elements incorporated into proteins through a dedicated genetic codon — selenocysteine is sometimes called the twenty-first amino acid.
• The Keshan disease, a sometimes fatal heart condition, was traced in the 1970s to severe selenium deficiency in soils in parts of rural China, establishing selenium's role as an essential nutrient.
• At room temperature selenium exists in several allotropic forms, the most stable being grey trigonal selenium, which has a helical crystal structure and is the best electrical conductor of the group.
• Chester Carlson, the inventor of xerography, spent years being rejected by companies including IBM and RCA before Haloid (later Xerox) licensed his selenium-based copying process.
• Pure selenium emits a distinctive odor described as resembling rotten radishes, which serves as a useful if unpleasant warning of exposure to unsafe concentrations.