There are a number of allotropic forms, including grey, red, and black selenium. It occurs in sulphide ores of other metals and is obtained as a by-product (e.g. from the anode sludge in electrolytic refining). The element is a semiconductor; the grey allotrope is light-sensitive and is used in photocells, xerography, and similar applications.
Properties: at.no. 34; r.a.m. 78.96; r.d. 4.79; m.p. 217°C (422.6°F); b.p. 684.9°C (1265°F); most common isotope Se80 (49.82%).
