Element Tellurium, Te, Metalloid

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 HTe'. 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, H₂TeO₃, 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(OH)₄ = H₂TeO₃ + 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.

Tellurous acid can be converted into telluric acid, H₂TeO₄, by powerful oxidising agents. This has no similarity to sulphuric or selenic acid, for it forms a crystalline mass, sparingly soluble in water, and possessing feebly acid properties. The crystals which can be obtained from water have the composition H₂TeO₄ + 2H₂O = Te(OH)₆, and, on heating, pass first into the acid H₂TeO₄, and then into the anhydride TeO₃. The latter is a yellow mass, which is indifferent towards water.

Telluric acid also exhibits basic properties.

Whereas, therefore, the oxygen compounds of tellurium agree in composition with those of sulphur and selenium, their chemical behaviour is very different. It is, it is true, a general phenomenon that the elements with higher combining weight form more feebly acid or more strongly basic compounds compared with the related elements of lower combining weight, but it is seldom that the phenomenon makes its appearance so abruptly as in the present case.

Finally, it has to be mentioned that tellurium combines with chlorine to form the compounds TeCl₂ and TeCl₄, and with bromine to form TeBr₂ and TeBr₄. These are crystalline substances, volatile without decomposition at moderately high temperatures, and resembling in properties the halogen compounds of the metals, e.g. of mercury.

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 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) PbTe, and hessite (telluric silver) Ag₂Te, sylvanite AuAgTe₄, calaverite AuTe₂, 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).