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Chemical elements
  Tellurium
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    PDB 1el7-3l1k

Element Tellurium, Te, Metalloid

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While iodine is very similar to bromine, if not in external appearance, at least in the nature of the corresponding compounds, the differences between selenium and tellurium are more pronounced, so that it has repeatedly been regarded as doubtful whether these two elements ought to be regarded as members of the same family.

Tellurium is a greyish white substance with a metallic lustre; its density is 6.4, and it conducts the electric current like a metal. It melts at 450°, and boils at 1400°. The vapour is only slightly denser than corresponds to the formula Te₂.

The combining weight of tellurium, Te = 127.6, is, as has already been mentioned, greater than that of iodine, whereas the combining weights of the other elements of this group are smaller than those of the corresponding halogens. Many investigations have therefore been carried out because it was believed that this deviation was due to incorrect determinations; it has, however, been found with certainty that the numbers are as stated.

Besides the metallic tellurium, there is also a black, apparently amorphous form, in which tellurium is obtained by precipitation from its solutions. It has a considerably smaller density.

Tellurium combines with the metals to form tellurides; these correspond to the sulphides in composition, and have generally a metallic appearance. The tellurides of the alkali metals are soluble in water, and form the ions Te'' and H Te'. From these tellurium separates out under the influence of the oxygen of the air.

Tellurium hydride, H₂Te, is obtained by the action of the strong acids on tellurides; it is a gas possessing an offensive smell and poisonous properties, behaves similarly to the hydrogen compounds of sulphur and selenium, and in aqueous solution is, like these, readily decomposed by the oxygen of the air.

Heated in the air, tellurium burns, forming the dioxide, which is a white substance volatile at a red heat. Tellurous acid, H₂Te O₃, is obtained by oxidising tellurium with nitric acid. It is a white mass, slightly soluble in water, and has only feebly acid properties. With strong acids, the compound behaves as a base, hydroxyl being eliminated and a salt formed. Compounds are hereby produced which are derived from a tetracid base, Te(O H)₄ = H₂Te O₃ + H₂O. These salts are, however, also very unstable towards water, just as, in fact, all substances which can act both as base and as acid yield in both cases only slightly stable compounds.

Tellurium History

Tellurium had been discovered when analytical chemistry began flourishing in the second half of 18^th century. A new gold-containing ore had been found in Siebenburgen (Transylvania). Franz-Joseph Muller (later baron von Reichenstein), chief inspector of mines in Siebenburgen, analyzed the ore and extracted from it a substance considered that time as a new metal. Muller sent samples to Torbern Bergman, the Swedish chemist-analyst for further analysis, but Bergman died before he could do further analysis. In the meantime Paul Kitaibel, the Hungarian professor of botany and chemistry, in 1789 had extracted some unknown, as he reckoned, metal (actually metalloid) from wehrlit. Kitaibel provided the detailed description of new-discovered element; however he did not report about his discovery, just sent letters to some scientists. One of them was received by F.J.A. Estner, a mineralogist from Vienna, and he informed Klaproth about it, Martin Heinrich Klaproth (1743-1817). He succeeded and confirmed the discovery of the new metal. In his paper before the Academy of Sciences of Berlin of 25 January 1798 he gave the metal the "von der alten Mutter Erde entlehnten" name Tellurium that derives of Latin tellus = Earth.

Tellurium Occurrence

Tellurium is present in the Earth's crust at an average concentration of only about 1 x 10^-7% by mass. It is a constituent part of 100 minerals including native Tellurium, altaite (lead telluride) Pb Te, and hessite (telluric silver) Ag₂Te, sylvanite Au Ag Te₄, calaverite Au Te₂, tetradymite (bismuth telluride) Bi₂Te₂S. As an isomorphous mixture tellurium is found in chalcopyrite, galena, pyrite and pentlandite. Tellurium may be recovered from copper, lead and pyrite ores in which it is dispersed as a trace element or as microinclusions of its own minerals. Some of tellurium own minerals also may contain gold which, however, does not have any commercial significant.

Tellurium is permanently present in plant and animal tissues. An average concentration in plants growing on tellurium-rich soils reaches 2x10^-4-2.5x10^-3%, in terrestrial animal organisms - 2x10^-6%. Daily supply with food and water is 0.6 mg. Tellurium is moderately toxic for plants and highly poisonous for mammals it may cause inhibition of growth, fall-off, paralysis and so forth).

Neighbours

Chemical Elements
³³As 74.9 Arsenic	³⁴Se 79.0 Selenium	³⁵Br 79.9 Bromine
⁵¹Sb 121.8 Antimony	⁵²Te 127.6 Tellurium	⁵³I 126.9 Iodine
⁸³Bi 209.0 Bismuth	⁸⁴Po [210.0] Polonium	⁸⁵At [210.0] Astatine