Tellurium: the essentials
Crystalline tellurium has a silvery-white appearance, and exhibits a metallic lustre when pure (see above). It is brittle and easily pulverised. Tellurium is a p-type semiconductor, and shows varying conductivity with crystal alignment. Its conductivity increases slightly with exposure to light. It can be doped with silver, copper, gold, tin, or other elements.
Humans exposed to as little as 0.01 mg m-3 in air, or less, develop "tellurium breath", which has a garlic-like odour.
Tellurium: historical information
Tellurium was discovered in gold ores by Franz Joseph Mδller von Reichenstein, the chief inspector of mines in Trannsylvania in 1782. However tellurium was named by M. Klaproth, who isolated it in 1798, after he continued Mδller von Reichenstein's work.
Tellurium: physical properties
Tellurium: orbital properties
Isolation: it is not usually necessary to make tellurium in the laboratory as it is commercially available. While there are some tellurium ores, most tellurium is made as a byproduct of copper refining. Extraction is complex since the method emplyed will depend upon what other compounds or elements are present. The first step usually involves an oxidation in the presence of sodium carbonate (soda ash).
Cu2Te + Na2CO3 + 2O2 → 2CuO + Na2TeO3 + CO2
The tellurite Na2TeO3 is acidified with sulphuric acid and the tellurium precipitates out as the dioxide (leaving and selenous acid, H2SeO3, in solution). Tellurium is liberated from the dioxide by dissolving in sodium hydroxide, NaOH, and electroytic reduction.
TeO2 + 2NaOH → Na2TeO3 + H2O → Te + 2NaOH + O2