Selenium Selenium Selenium is a chemical element with symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, and also has similarities to arsenic. It rarely occurs in its elemental state or as pure ore compounds in the Earth's crust. Selenium (from Ancient Greek (selḗnē) "Moon") was discovered in 1817 by Jöns Jacob Berzelius, who noted the similarity of the new element to the previously discovered tellurium (named for the Earth). Selenium is found in , where it
Arsenic pentachloride Arsenic pentachloride Arsenic pentachloride is a chemical compound of arsenic and chlorine. This compound was first prepared in 1976 through the UV irradiation of arsenic trichloride, AsCl, in liquid chlorine at −105 °C. AsCl decomposes at around −50 °C. The structure of the solid was finally determined in 2001. AsCl is similar to phosphorus pentachloride, PCl in having a trigonal bipyramidal structure where the equatorial bonds are shorter than the axial bonds (As-Cl = 210.6 pm, 211.9 pm; As-Cl= 220.7 pm). The pentachlorides of the elements above and below arsenic in group 15, phosphorus pentachloride and antimony pentachloride are much