Tellurium
Tellurium was discovered in Transylvania (today part of Romania) in 1782 by Franz-Joseph Müller von Reichenstein in a mineral containing tellurium and gold. Martin Heinrich Klaproth named the new element in 1798 after the Latin word for "earth", tellus. Gold telluride minerals are the most notable natural gold compounds. However, they are not a commercially significant source of tellurium itself, which is normally extracted as a by-product of copper and lead production.
Commercially, the primary use of tellurium is in alloys, foremost in steel and copper to improve machinability. Applications in solar panels and as a semiconductor material also consume a considerable fraction of tellurium production.
Tellurium has no biological function, although fungi can incorporate it in place of sulfur and selenium into amino acids such as tellurocysteine and telluromethionine. In humans, tellurium is partly metabolized into dimethyl telluride, (CH3)2Te, a gas with a garlic-like odor which is exhaled in the breath of victims of tellurium toxicity or exposure.
Occurrence
With an abundance in the Earth's crust comparable to that of platinum, tellurium is one of the rarest stable solid elements in the Earth's crust. Its abundance is about 1 µg/kg. In comparison, even the rarest of the lanthanides have crustal abundances of 500 µg/kg (see Abundance of the chemical elements).
The extreme rarity of tellurium in the Earth's crust is not a reflection of its cosmic abundance, which is in fact greater than that of rubidium, even though rubidium is ten thousand times more abundant in the Earth's crust. The extraordinarily low abundance of tellurium on Earth is rather thought to be due to conditions in the Earth's formation, when the stable form of certain elements, in the absence of oxygen and water, was controlled by the reductive power of free hydrogen. Under this scenario, certain elements such as tellurium which form volatile hydrides were severely depleted during the formation of the Earth's crust, through evaporation of these hydrides. Tellurium and selenium are the heavy elements most depleted in the Earth's crust by this process.
Tellurium is sometimes found in its native (i.e., elemental) form, but is more often found as the tellurides of gold such as calaverite and krennerite (two different polymorphs of AuTe2), petzite, Ag3AuTe2, and sylvanite, AgAuTe4. The city of Telluride, Colorado was named in hope of a strike of gold telluride (which never materialized, though gold metal ore was found). Gold itself is usually found uncombined, but when found naturally as a chemical compound, it is most often combined with tellurium (a few rare non-telluride gold compounds such as the antimonide aurostibite, AuSb2, and bismuthide maldonite, Au2Bi, are also known).
Although tellurium is found with gold more often than in uncombined form, it is found even more often combined with elements other than gold, as tellurides more common metals (e.g. melonite, NiTe2). Natural tellurite and tellurate minerals also occur, formed by oxidation of tellurides near the Earth's surface. In contrast to selenium, tellurium is not in general able to replace sulfur in its minerals, due to the large difference in ion radius of sulfur and tellurium. In consequence, many common sulfide minerals contain considerable amounts of selenium, but only traces of tellurium.
In the gold rush of 1893, diggers in Kalgoorlie discarded a pyritic material which got in their way as they searched for pure gold. The Kalgoorlie waste was thus used to fill in potholes or as part of sidewalks. Three years passed before it was realized that this waste was calaverite, a telluride of gold that had not been recognized. This led to a second gold rush in 1896 which included mining the streets.
| Symbol | Te | |
| Atomic Number | 52 | |
| Atomic Weight | 127.6 | |
| Oxidation States | +4,+6,-2 | |
| Electronegativity, Pauling | 2.1 | |
| State at RT | Solid, Metalloid | |
| Melting Point, K | 722.7 | |
| Boiling Point, K | 1263 |
Appearance and Characteristics
Harmful effects:
Tellurium is very toxic and teratogenic (can cause harm to developing embryos). Exposure to as little as 0.01 mg/m2 or less in air leads to “tellurium breath”, which has a garlic-like odor.
Characteristics:
- Tellurium is a rare, silvery-white, brittle, lustrous metalloid.
- It burns in air with a greenish-blue flame and forms white tellurium dioxide (TeO2).
- When present in compounds, tellurium exists mostly in the oxidation state IV and VI.
- Tellurium is a semiconductor material and is slightly photosensitive.
- It forms many compounds corresponding to those of sulfur and selenium, the elements above it in the periodic table. For example, it forms tellurides with other metals and tellurite (TeO32-) and tellurate (TeO42-) compounds.
- Tellurium has radioactive isotopes and is the lightest element to exhibit alpha decay.
Uses of Tellurium
- Tellurium is alloyed with copper and stainless steel to make these metals more workable.
- It is added at very low levels to lead to decreases the corrosive action of sulfuric acid in batteries and to improve the lead’s strength and hardness.
- Tellurium is used as a coloring agent in ceramics.
- Tellurium is also used in the electronics industry, for example with cadmium and mercury to form photosensitive semiconductors. Cadmium telluride (CdTe) is used as a thin film in solar panels to convert sunlight into electricity. CdTe panels have an efficiency rating of between 11 and 13 percent compared to amorphous silicon solar panels which have an efficiency of between 7 to 9 percent. (6)
- It is used in vulcanizing rubber and in catalysts for petroleum cracking and in blasting caps for explosives.
