Selenium
. Selenium (Greek selēnē, “moon”), symbol Se, semimetallic element with an atomic number of 34. Selenium is in group 16 (or VIa) of the periodic table . Selenium was discovered in 1817 by the Swedish chemist Baron Jöns Jakob Berzelius in a sulfuric acid residue. It was so called because it was found in association with tellurium (Latin tellus, “earth”). PROPERTIES AND OCCURRENCE Chemically, selenium closely resembles sulfur and is related to tellurium. Like sulfur, it exists in several allotropic (distinctly different) forms: a brick-red powder; a brownish-black, glassy, amorphous mass called vitreous selenium; red monoclinic crystals of specific gravity 4.5; and gray, lustrous crystals called gray selenium. It forms selenious acid and selenic acid, the respective salts of which are called selenites and selenates. Gray selenium melts at 217°C (423°F), boils at about 685°C (about 1265°F), and has a specific gravity of 4.81. The atomic weight of selenium is 78.96. The element occurs in