Selenium is a metal-like element signified by the atomic symbol Se. The average atomic weight of its isotopes is 78.96, and its atomic number is 34. The element exists in at least three allotropic forms, which may be either amorphous or crystalline and red or black in color. Both its chemical and physical properties are similar to those of sulfur, the element just above it in the periodic table.

Selenium was discovered in 1817 by the famous Swedish chemist, Jöns Berzelius. Berzelius found the element in the sludge deposited at the bottom of a tank in a sulfuric acid factory. In a letter to a friend dated September 23, 1817, Berzelius described how some of the sulfur burned in the factory escaped oxidation and, instead, deposited at the bottom of the lead chamber. He noted that this sulfur-based sludge also seemed to contain a small amount of tellurium, an element discovered...