tellurium (tĕlŏrˈēəm) [key] [Lat., = earth], semimetallic chemical element; symbol Te; at. no. 52; at. wt. 127.60; m.p. 450°C; b.p. 990°C; sp. gr. 6.24 at 20°C; valence - 2, +4, or +6. Tellurium is a lustrous, brittle, crystalline, silver-white metalloid. A powdery brown form of the element is also known. Tellurium forms many compounds corresponding to those of sulfur and selenium, the elements above it in Group 16 of the periodic table. The dioxide, TeO2, is formed when the element is burned in air. Tellurium forms two weak acids and a number of halogen compounds. With hydrogen and with some metals it forms tellurides. Tellurium and its compounds are probably poisonous. Tellurium is occasionally found uncombined in nature but is more often found combined with metals, as in the minerals calaverite (gold telluride) and sylvanite (silver-gold telluride). Tellurium is recovered as a byproduct of the electrolytic refining of blister copper. It is used as an additive to steel and is often alloyed with aluminum, copper, lead, or tin. It is used in vulcanizing rubber, as a coloring agent in glass and ceramics, and in catalysts for petroleum cracking. Tellurium is a semiconductor material and is slightly photosensitive. It is used with bismuth in thermoelectric devices. Tellurium was discovered in 1782 by Franz Muller von Reichenstein. It was named by M. H. Klaproth, who isolated it in 1798.