A nonmetallic element of the sulphur group, and analogous to sulphur in its compounds. It is found in small quantities with sulphur and some sulphur ores, and obtained in the free state as a dark reddish powder or crystalline mass, or as a dark metallic-looking substance. It exhibits under the action of light a remarkable variation in electric conductivity, and is used in certain electric apparatus. Symbol Se. Atomic weight 78.9.
- A nonmetallic element, red in powder form, black in vitreous form, and metallic gray in crystalline form, resembling sulfur and obtained primarily as a byproduct of electrolytic copper refining. It is widely used in rectifiers, as a semiconductor, and in xerography. Its photovoltaic and photoconductive actions make it useful in photocells, photographic exposure meters, and solar cells. Atomic number 34; atomic weight 78.96; melting point (of gray selenium) 217°C; boiling point (gray) 684.9°C; specific gravity (gray) 4.79; (vitreous) 4.28; valence 2, 4, or 6. See Table at element.