Selenium

Selenium is a nonmetal in group 16, directly below sulfur, with six valence electrons ([Ar]3d10 4s2 4p4). It forms selenides (−2) and compounds in the +4 and +6 states, much as sulfur does. Selenium has several allotropes: red amorphous powder, black glassy selenium, and the stable gray form made of spiral chains of atoms. Gray selenium is a semiconductor whose electrical conductivity rises sharply in light, a property discovered in 1873 that led to early light meters, the first solid-state solar cell and the photocopier.

Selenium is both essential and poisonous, with a narrow window between the two. Adults need about 55 micrograms a day, while regular intakes above about 400 micrograms risk poisoning. The body builds selenium into the amino acid selenocysteine, found in antioxidant enzymes and in the enzymes that activate thyroid hormone. Industrially, selenium is used in glassmaking, in the electrolytic production of manganese, and in thin-film copper indium gallium selenide solar cells.

Discovery

In 1817 the Swedish chemists Jöns Jacob Berzelius and Johan Gottlieb Gahn noticed a reddish sludge in the lead chambers of a sulfuric acid works that used pyrite from the Falun copper mine. At first they thought it contained tellurium, but Berzelius showed it was a new element. Because tellurium had been named after the Earth (Latin tellus), he named its chemical cousin after the Moon, from the Greek selēnē.