Selenium is directly below sulfur in group VIa of the periodic table .
In chemical activity and physical properties it resembles sulfur and tellurium. 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. 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.