word-forming element, from Latin se-, collateral form of sed- "without, apart, aside, on one's own," related to sed, Latin reflexive pronoun (accusative and ablative), from PIE *sed-, extended form of root *s(w)e-, pronoun of the third person and reflexive (cf. German sich; see idiom).
A nonmetallic element that occurs in a gray crystalline form, as a red powder, or as a black glassy material. It is highly photosensitive and can be used to convert light into electricity. Its ability to conduct electricity also increases with higher exposure to light. For these reasons selenium is used in photocopying technology, photography, and solar cells. Atomic number 34; atomic weight 78.96; melting point 217°C; boiling point 684.9°C; specific gravity (gray) 4.79; (red) 4.5; (black) 4.28; valence 2, 4, or 6. See Periodic Table.
