selenium (səlē´nēəm), nonmetallic chemical element; symbol Se; at. no. 34; at. wt. 78.96; m.p. 217°C; b.p. about 685°C; sp. gr. 4.81 at 20°C; valence -2, +4, or +6. Selenium is directly below sulfur in Group 16 of the periodic table. In chemical activity and physical properties it resembles sulfur and tellurium. Selenium exhibits allotropy, appearing in a number of forms, including a red amorphous powder, a red crystalline material, and a gray crystalline metallike form called "metallic" selenium. A remarkable property (discovered by Willoughby Smith in 1873) of the gray metallic form is that its electrical conductivity is greater in light than in darkness, and it increases as the illumination increases. This property has led to use of the metallic form in the junction rectifier and as a cathode in the photoelectric cell rectifier. Selenium is extensively used in the vulcanization of rubber, in the manufacture of red glass and some enamels, as a decolorizer of glass to counteract the green of iron compounds, in electronics, and in xerography. Selenium forms the oxides SeO2 and SeO3, the selenious (H2SeO3) and selenic (H2SeO4) acids and their respective selenite and selenate salts, a nitride, carbide, hydride, two sulfides, and various halides and oxyhalides. Selenium sometimes occurs in conjunction with sulfur deposits and often occurs as the selenide (especially of copper, lead, silver, and iron) in sulfide ores. Commercially it is obtained chiefly as a byproduct in the refining of copper. In the Great Plains region and certain other areas, selenium is absorbed from the soil by vegetation in quantities sufficient to poison livestock, thus rendering the land useless for grazing. Nonetheless, selenium is one of the elements needed in trace amounts in the animal and human diet. Fish, meat, poultry, whole grains, and dairy products are good sources of this mineral nutrient in the human diet. The element was discovered by Berzelius in 1817.