Selenium is a Block P, Group 16, Period 4 element. The number of electrons in each of Selenium's shells is 2, 8, 18, 6 and its electronic configuration is [Ar] 3d¹⁰ 4s² 4p⁴. In its elemental form selenium's CAS number is 7782-49-2. The selenium atom has a radius of 116.pm and it's Van der Waals radius is 190.pm. The EPA does not classify selenium as carcinogenic, although selenium sulfide is a probable carcinogen. Selenates and selenites which are compounds of selenium, are highly toxic. Hydrogen selenide gas (SeH2) is the most acutely toxic compound of selenium. Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. of photocells and exposure meters for photographic use, as well as solar cells. Below its melting point, selenium is a p-type semiconductor and has many uses in electronic and solid-state applications. Selenium is available as metal and compounds with purities from 99% to 99.9999% (ACS grade to ultra-high purity); metals in the form of foil, sputtering target, and rod, and compounds as submicron and nanopowder. Selenium was first discovered by Jons Berzelius in 1817. The origin of the name Selenium comes from the Greek word "Selênê" meaning moon. See selenium research below.