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antimonytelluriumiodine
Se

Te

Po
Appearance
silvery lustrous gray
General properties
Name, symbol, number tellurium, Te, 52
Element category metalloid
Group, period, block 165, p
Standard atomic weight 127.60g·mol−1
Electron configuration [Kr] 4d10 5s2 5p4
Electrons per shell 2, 8, 18, 18, 6 (Image)
Physical properties
Phase solid
Density (near r.t.) 6.24 g·cm−3
Liquid density at m.p. 5.70 g·cm−3
Melting point 722.66 K, 449.51 °C, 841.12 °F
Boiling point 1261 K, 988 °C, 1810 °F
Heat of fusion 17.49 kJ·mol−1
Heat of vaporization 114.1 kJ·mol−1
Specific heat capacity (25 °C) 25.73 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K     (775) (888) 1042 1266
Atomic properties
Oxidation states 6, 5, 4, 2, -2
(mildly acidic oxide)
Electronegativity 2.1 (Pauling scale)
Ionization energies 1st: 869.3 kJ·mol−1
2nd: 1790 kJ·mol−1
3rd: 2698 kJ·mol−1
Atomic radius 140 pm
Covalent radius 138±4 pm
Van der Waals radius 206 pm
Miscellanea
Crystal structure hexagonal
Magnetic ordering diamagnetic[1]
Thermal conductivity (300 K) (1.97–3.38) W·m−1·K−1
Speed of sound (thin rod) (20 °C) 2610 m/s
Young's modulus 43 GPa
Shear modulus 16 GPa
Bulk modulus 65 GPa
Mohs hardness 2.25
Brinell hardness 180 MPa
CAS registry number 13494-80-9
Most stable isotopes
Main article: Isotopes of tellurium
iso NA half-life DM DE (MeV) DP
120Te 0.09% >2.2×1016y ε ε 1.701 120Sn
121Te syn 16.78 d ε 1.040 121Sb
122Te 2.55% 122Te is stable with 70 neutrons
123Te 0.89% >1.0×1013 y ε 0.051 123Sb
124Te 4.74% 124Te is stable with 72 neutrons
125Te 7.07% 125Te is stable with 73 neutrons
126Te 18.84% 126Te is stable with 74 neutrons
127Te syn 9.35 h β 0.698 127I
128Te 31.74% 2.2×1024 y ββ 0.867 128Xe
129Te syn 69.6 min β 1.498 129I
130Te 34.08% 7.9×1020 y ββ 2.528 130Xe

Tellurium (pronounced /tɪˈlʊəriəm, tɛ-/ te-LOOR-ee-əm) is a chemical element that has the symbol Te and atomic number 52. A brittle, mildly toxic, silver-white metalloid which looks similar to tin, tellurium is chemically related to selenium and sulfur. Tellurium was discovered in 1782 by Franz-Joseph Müller von Reichenstein in a mineral containing gold and tellurium. Martin Heinrich Klaproth named the new element in 1798 after the Latin word for "earth", tellus. Although several gold deposits contain tellurium minerals, the main commercial source for tellurium is as a by-product of copper and lead production. Tellurium is primarily used 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 telluro-cysteine and telluro-methionine. 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.

Contents

History

Oval black and white engraving of a man looking left with a scarf and a coat with large buttons.
Klaproth named the new element and credited von Reichenstein with its discovery

Tellurium (Latin tellus meaning "earth") was discovered in the 18th century in a gold ore from the mines in Zlatna, near what is now Sibiu, Transylvania. This ore was known as "Faczebajer weißes blättriges Golderz" (white leafy gold ore from Faczebaja) or antimonalischer Goldkies (antimonic gold pyrite), and, according to Anton von Rupprecht, was Spießglaskönig (argent molybdique), containing native antimony.[2] In 1782 Franz-Joseph Müller von Reichenstein, who was then serving as the Hungarian chief inspector of mines in Transylvania, concluded that the ore did not contain antimony, but that it was bismuth sulfide.[3] The following year, he reported that this was erroneous and that the ore contained mostly gold and an unknown metal very similar to antimony. After a thorough investigation which lasted for three years and consisted of more than fifty tests, Müller determined the specific gravity of the mineral and noted the radish-like odor of the white smoke which passed off when the new metal was heated, the red color which the metal imparts to sulfuric acid, and the black precipitate which this solution gives when diluted with water. Nevertheless, he was not able to identify this metal and gave it the names aurum paradoxium and metallum problematicum, as it did not show the properties predicted for the expected antimony.[4][5][6]

In 1789, another Hungarian scientist, Pál Kitaibel, also discovered the element independently in an ore from Deutsch-Pilsen which had been regarded as argentiferous molybdenite, but later he gave the credit to Müller. In 1798, it was named by Martin Heinrich Klaproth who earlier isolated it from the mineral calaverite.[5][6][7]

Tellurium was used as a chemical bonder in the making of the outer shell of the first atomic bomb. The 1960s brought growth in thermoelectric applications for tellurium, as well as its use in free-machining steel, which became the dominant use.

Characteristics

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Physical

When crystalline, tellurium is silvery-white and when it is in pure state it has a metallic luster. It is a brittle and easily pulverized metalloid. Amorphous tellurium is found by precipitating it from a solution of tellurous or telluric acid (Te(OH)6).[8] Tellurium is a p-type semiconductor that shows a greater electrical conductivity in certain directions which depends on atomic alignment; the conductivity increases slightly when exposed to light (photoconductivity).[9] When in its molten state, tellurium is corrosive to copper, iron and stainless steel.

Isotopes

Naturally occurring tellurium has eight isotopes. Four of those isotopes, 122Te, 124Te, 125Te and 126Te, are stable. The other four, 120Te, 123Te, 128Te and 130Te, have been observed to be radioactive.[10][11] The stable isotopes make up only 33.2 % of the naturally occurring tellurium; this is possible due to the long half-lives of the unstable isotopes. They are in the range from 1013 to 2.2 1024 years. This makes 128Te the isotope with the longest half life among all radioisotopes.[12]

There are 38 known nuclear isomers of tellurium with atomic masses that range from 105 to 142. Tellurium is the lightest element known to undergo alpha decay, with isotopes 106Te to 110Te being able to undergo this mode of decay.[10] The atomic mass of tellurium (127.60 g·mol−1) exceeds that of the following element iodine (126.90 g·mol−1).[13]

Occurrence

A dark mass, approximately 2 millimetres in diameter, on a rose-coloured crystal substrate
Tellurium on quartz (Moctezuma, Sonora, Mexico)

With an abundance in the Earth's crust comparable to that of platinum, tellurium is one of the rarest stable solid element in the Earth's crust. Its abundance is about 1 µg/kg.[14] In comparison, even the rarest of the lanthanides have crustal abundances of 500 µg/kg (see Abundance of the chemical elements).[15]

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.[citation needed]

Tellurium is sometimes found in its native (elemental) form, but is more often found as the tellurides of gold (calaverite, krennerite, petzite, sylvanite and others). Tellurium compounds are the most common chemical compounds of gold found in nature (rare non-tellurides such as gold aurostibite and bismuthide are known). Tellurium is also found combined with elements other than gold, in salts of other metals. In contrast to selenium, tellurium is not able to replace sulfur in its minerals. This is due to the large difference in ion radius of sulfur and tellurium. In consequence, many sulfide minerals contain considerable amounts of selenium, but only traces of tellurium.[16]

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 on 29 May 1896.[17]

Compounds and chemistry

Tellurium belongs to the same chemical family as oxygen, sulfur, selenium and polonium: the chalcogen family. It forms similar compounds to sulfur and selenium. It exhibits the oxidation states −2, +2, +4 and +6, with the +4 state being most common.[8]

Tellurides

The −2 oxidation state is exhibited in binary compounds with many metals, such as zinc telluride, ZnTe, formed by heating tellurium with zinc.[18] Decomposition of ZnTe with hydrochloric acid yields hydrogen telluride, H2Te, the tellurium analogue of the other chalcogen hydrides, H2O, H2S and H2Se:

ZnTe + 2 HCl → ZnCl2 + H2Te

H2Te reacts with many metals to form tellurides, containing the Te2− anion.[18] Gold and silver tellurides are considered good ores.

Halides

The +2 oxidation state is exhibited by the monoxide, TeO, and the dihalides, TeCl2, TeBr2 and TeI2. The dihalides have not been obtained in pure form,[19]:274 although they are known decomposition products of the tetrahalides in organic solvents, and their derived tetrahalotellurates are well-characterized:

Te + X2 + 2 XTeX2−4

where X is Cl, Br, or I. These anions are square planar in geometry.[19]:281 Polynuclear anionic species also exist, such as the dark brown Te2I2−6,[19]:283 and the black Te4I2−14.[19]:285

Fluorine forms two halides with tellurium: the mixed-valence Te2F4 and TeF6. In the +6 oxidation state, the –OTeF5 structural group occurs in a number of compounds such as HOTeF5, B(OTeF5)3, Xe(OTeF5)2, Te(OTeF5)4 and Te(OTeF5)6.[20] The square antiprismatic anion TeF2−8 is also attested.[21] The other halogens do not form halides with tellurium in the +6 oxidation state, but only tetrahalides (TeCl4, TeBr4 and TeI4) in the +4 state, and other lower halides (Te3Cl2, Te2Cl2, Te2Br2, Te2I and two forms of TeI). In the +4 oxidation state, halotellurate anions are known, such as TeCl2−6 and Te2Cl2−10. Halotellurium cations are also attested, including TeI+3, found in TeI3AsF6.[22]

Oxocompounds
A sample of pale yellow powder
A sample of tellurium dioxide powder

Tellurium monoxide was first reported in 1883 as a black amorphous solid formed by the heat decomposition of TeSO3 in vacuum, disproportionating into tellurium dioxide, TeO2 and elemental tellurium upon heating.[23][24] Since then, however, some doubt has been cast on its existence in the solid phase, although it is known as a vapor phase fragment; the black solid may be merely an equimolar mixture of elemental tellurium and tellurium dioxide.[25]

Tellurium dioxide is formed by heating tellurium in air, causing it to burn with a blue flame.[18] Tellurium trioxide, β-TeO3, is obtained by thermal decomposition of Te(OH)6. The other two forms of trioxide reported in the literature, the α- and γ- forms, were found not to be true oxides of tellurium in the +6 oxidation state, but a mixture of Te4+, OH and O2.[26] Tellurium also exhibits mixed-valence oxides, Te2O5 and Te4O9.[26]

The tellurium oxides and hydrated oxides form a series of acids, including tellurous acid (H2TeO3), orthotelluric acid (Te(OH)6) and metatelluric acid ((H2TeO4)n).[25] The two forms of telluric acid form tellurate salts containing the TeO2–4 and TeO6−6 anions, respectively. Tellurous acid forms tellurite salts containing the anion TeO2−3. Other tellurium cations include TeF2+8, which consists of two fused tellurium rings and the polymeric TeF2+7.

Zintl cations

When tellurium is treated with concentrated sulfuric acid, it forms red solutions containing the Zintl ion, Te2+4. The oxidation of tellurium by AsF5 in liquid SO2 also produces this square planar cation, as well as with the trigonal prismatic, yellow-orange Te4+6:[21]

4 Te + 3 AsF5Te2+4(AsF6)2 + AsF3
6 Te + 6 AsF5Te4+6(AsF6)4 + 2 AsF3

Other tellurium Zintl cations include the polymeric Te2+7 and the blue-black Te2+8, which consists of two fused 5-membered tellurium rings. The latter cation is formed by the reaction of tellurium with tungsten hexachloride:[21]

8 Te + 2 WCl6Te2+8(WCl6)2

Interchalcogen cations also exist, such as Te2Se2+6 (distorted cubic geometry) and Te2Se2+8. These are formed by oxidizing mixtures of tellurium and selenium with AsF5 or SbF5.[21]

Organotellurium compounds

In organic chemistry, tellurium forms analogues of alcohols and thiols, which have the functional group –TeH and are called tellurols. The –TeH functional group is also attributed to using the prefix tellanyl-.[27]

Production

The principal source of tellurium is from anode sludges produced during the electrolytic refining of blister copper. It is a component of dusts from blast furnace refining of lead. Treatment of 500 tons of copper ore typically yields one pound (0.45 kg) of tellurium. Tellurium is produced mainly in the United States, Canada, Peru and Japan.[28] For the year 2006 the British Geological Survey gives the following numbers: Canada 11 t, United States 50 t, Peru 37 t and Japan 24 t.[29]

Grey and white world map with four countries colored to show the percentage of world wide tellurium production. US to produce 40%; Peru to produce 30%; Japan to produce 20%;and Canada to produce 10%
Tellurium production 2006

The anode sludges contain the selenides and tellurides of the noble metals in compounds with the formula M2Se or M2Te (M = Cu, Ag, Au). At temperatures of 500 °C the anode sludges are roasted with sodium carbonate under air. The metals are reduced to the metals, while the tellurium is converted to sodium tellurite.[21]

M2Te + O2 + Na2CO3 → Na2TeO3 + 2 M + CO2

Tellurites can be leached from the mixture with water and are normally present as hydrotellurites HTeO3- in solution. Selenates are also formed during this process, but they can be separated by adding sulfuric acid. The hydrotellurites are converted into the insoluble tellurium dioxide while the selenites stay in solution.[21]

HTeO3- + OH- + H2SO4 → TeO2 + 2 SO42− + 2 H2O

The reduction to the metal is done either by electrolysis or by reacting the tellurium dioxide with sulfur dioxide in sulfuric acid.[21]

TeO2 + 2 SO2 + 2H2O → Te + SO42− + 4 H+

Commercial-grade tellurium is usually marketed as minus 200-mesh powder but is also available as slabs, ingots, sticks, or lumps. The year-end price for tellurium in 2000 was US$14 per pound. In recent years, the tellurium price was driven up by increased demand and limited supply, reaching as high as US$100 per pound in 2006.[30][31]

Applications

Metallurgy

The largest consumer of tellurium is metallurgy, where it is used in iron, copper and lead alloys. When added to stainless steel and copper it makes these metals more machinable. It is alloyed into cast iron for promoting chill for spectroscopic purposes, as the presence of electrically conductive free graphite tends to deleteriously effect spark emission testing results. In lead it improves strength and durability and decreases the corrosive action of sulfuric acid.[32]

Semiconductor and electronic industry uses

Tellurium is used in cadmium telluride (CdTe) solar panels. National Renewable Energy Laboratory lab tests using this material achieved some of the highest efficiencies for solar cell electric power generation. Massive commercial production of CdTe solar panels by First Solar in recent years has significantly increased tellurium demand.[33][34] If some of the cadmium in CdTe is replaced by zinc then CdZnTe is formed which is used in solid-state x-ray detectors.[35]

Alloyed with both cadmium and mercury, to form mercury cadmium telluride, an infrared sensitive semiconductor material is formed.[36] Organotellurium compounds such as dimethyl telluride, diethyl telluride, diisopropyl telluride, diallyl telluride and methyl allyl telluride are used as precursors for Metalorganic vapor phase epitaxy growth of II-VI compound semiconductors. Diisopropyl telluride (DIPTe) is employed as the preferred precursor for achieving the low-temperature growth of CdHgTe by MOVPE.[37] For these process high purity metalorganics of both selenium and tellurium are used. The compounds for semiconductor industry and are prepared by adduct purification.[38][39]

Tellurium as a tellurium suboxide is used in the media layer of several types of rewritable optical discs, including ReWritable Compact Discs (CD-RW), ReWritable Digital Video Discs (DVD-RW) and ReWritable Blu-ray Discs.[40][41]

Tellurium is used in the new phase change memory chips[42] developed by Intel.[43] Bismuth telluride (Bi2Te3) is used in thermoelectric devices.

Other uses

Biological role

Tellurium has no biological function, although fungi can incorporate it in place of sulfur and selenium into amino acids such as telluro-cysteine and telluro-methionine. Organisms have show a highly variable tolerance on tellurium compounds. Most organisms metabolize tellurium partly to form dimethyl telluride although also dimethyl ditelluride is formed by some species. Dimethyl telluride has been observed in hot springs at very low concentrations.[50][51][52]

Precautions

Tellurium and tellurium compounds are considered to be mildly toxic and need to be handled with care, although acute poisoning is rare.[53] Tellurium is not reported to be carcinogenic.[53]

Humans exposed to as little as 0.01 mg/m3 or less in air develop "tellurium breath", which has a garlic-like odor.[44] The garlic odor that is associated with human intake of tellurium compounds is caused from the tellurium being metabolized by the body. When the body metabolizes tellurium in any oxidation state, the tellurium gets converted into dimethyl telluride, (CH3)2Te, which is volatile and is the cause of the garlic-like smell. Even though the metabolic pathways of tellurium are not known, it is generally assumed that they resemble those of the more extensively studied selenium, because the final methylated metabolic products of the two elements are similar.[54][55][56]

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External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

TELLURIUM [[[Symbol]] Te, atomic weight 127.5 (0=16)], a chemical element, found to a certain extent in nature in the uncombined condition, but chiefly in combination with other metals in the form of tellurides, such, for example, as sylvanite, black tellurium, and tetradymite. Small quantities are occasionally met with in iron pyrites, and hence tellurium is found with selenium in the flue dust, or chamber deposits of sulphuric acid works. Tellurium was first recognized as a distinct element in 1798 by M. H. Klaproth. It may be obtained by heating tellurium bismuth with sodium carbonate, lixiviating the fused mass with water, filtering, and exposing the filtrate to air, when the tellurium is gradually precipitated as a grey powder (J. J. Berzelius). J. Farbaky (Zeit. angew. Chem., 1897, p. II) extracts the element from black tellurium as follows:- The ore is boiled with concentrated sulphuric acid, the solution diluted, hydrochloric acid added and the tellurium (together with selenium) precipitated by sulphur dioxide and the process repeated when a purer tellurium is obtained. B. Brauner (Monats., 1889, 10, p. 414) recommends the following method for the purification. The crude element is treated with aqua regia and then evaporated with an excess of hydrochloric acid, the solution diluted and the tellurium precipitated by a current of sulphur dioxide. The precipitated tellurium is then fused with potassium cyanide, the melt extracted with water and the element precipitated by drawing a current of air through the solution and finally distilled in a current of hydrogen.

Tellurium is a brittle silvery-white element of specific gravity 6.27. It melts at 452° C. and boils at 478° C. (F. Krafft, Ber., 1903, 3 6, p. 4344). When heated in a current of hydrogen it sublimes in the form of brilliant prismatic crystals. An amor phous form is obtained when tellurium is precipitated from its solutions by sulphur dioxide, this variety having a specific gravity 6.015. When heated in air, tellurium burns, forming the dioxide Te02. The element is insoluble in water, but dissolves in concentrated sulphuric acid forming a deep red solution.

Like sulphur and selenium, tellurium combines directly with hydrogen to form telluretted hydrogen, TeH2, an extremely objectionable smelling and highly poisonous gas, which was first prepared by Sir H. Davy in 1810. It is best obtained by decomposing metallic tellurides with mineral acids. It is soluble in water, the solution gradually decomposing with deposition of tellurium; it also decomposes on exposure to light. It burns, and also, like sulphuretted hydrogen, precipitates many metals from solutions of their salts. It may be liquefied, the liquid boiling at o° C., and on further cooling, it solidifies, the solid melting at -48° C. Many tellurides of metals have been examined by C. A. Tibbals (Jour. Amer. Chem. Soc., 1909, 3 1, p. 902) who obtained the sodium and potassium tellurides by the direct union of their component elements and others from these by precipitation. The tellurides of the alkali metals immediately decompose on exposure to air, with liberation of tellurium. Two chlorides are known, the dichloride, TeC121 and the tetrachloride, TeCl 4. They are both obtained by passing chlorine over tellurium, the product being separated by distillation (the tetrachloride is the less volatile). The dichloride is an amorphous, readily fusible, almost black solid. It is decomposed by water with formation of tellurium and tellurous acid: 2TeC12+3H 2 0=Te+H 2 TeO 3 +4HC1. The tetrachloride is a white crystalline solid which is formed by the action of chlorine on the dichloride or by sulphur chloride on the element. It melts at 224° C. and is exceedingly hygroscopic. Water decomposes it with formation of tellurous acid and other products. It combines directly with sulphur trioxide to form a complex of composition TeC1 4.2SO 3. The tetrabromide similarly gives TeOBr2.2S03 (W. Prandtl, Zeit. anorg. Chem., 1909, 62, p. 237). Iodides are also known.

Two oxides of the element are definitely known, viz., the dioxide, Te02, and the trioxide, Te03, whilst a monoxide, TeO, has also been described. The dioxide is formed by burning tellurium in air or xxvi. 1 9 by warming it with nitric acid. It is a colourless crystalline solid which readily fuses to a yellow liquid. The trioxide is an orangecoloured solid which is formed when telluric acid is strongly heated. Tellurous acid, H 2 TeO 3, is obtained when the tetrachloride is decomposed by water, or on dissolving tellurium in nitric acid and pouring the solution into water. It is a colourless solid and behaves as a dibasic acid. The alkaline tellurites are soluble in water. It also gives rise to super-acid salts, such as KHTe03 H2Te03; K 2 TeO 3.3TeO 2. Telluric acid, H2Te04, is obtained in the form of its salts when tellurium is fused with potassium carbonate and nitre, or by the oxidizing action of chlorine on a tellurite in alkaline solution. The free acid may be obtained by decomposing the barium salt with sulphuric acid and concentrating the solution, when a crystalline mass of composition H 2 Te04.2H 2 O separates. It is also formed when the dioxide is oxidized by hydrogen peroxide in caustic potash solution (A. Gutbier, Zest. anorg. Chem., 1904, 40, p. 260), and perhaps best of all by oxidizing tellurium with a mixture of nitric and chromic acids. It crystallizes in prisms, which lose their water of crystallization at 160° C. The tellurates of the alkali metals are more or less soluble in water, those of the other metals being very sparingly or almost insoluble in water. Some tellurates exist in two forms, a colourless form soluble in water and acids, and a yellow form insoluble in water and acids. An oxychloride of tellurium has been described, but the investigations of V. Lenher (Jour. Amer. Chem. Soc., 1909, 31, p. 20) seem to negative its existence.

A considerable amount of work has been done on determinations of the atomic weight of tellurium, the earlier results giving the value 128. According to its position in the periodic classification of the elements one would expect its atomic weight to be less than that of iodine, instead of approximately equal, and on this account many efforts have been made to isolate another element from tellurium compounds, but none have as yet been successful. Recent investigations of the atomic weight are due to G. Gallo (Atti. R. Acad. Lincei, 1905 (iv.), 14, pp. I, 23, 104), who, by a determination of the electrochemical equivalent of the element, arrived at the value 127.61 A. Gutbier (Ann., 1905, 34 2, p. 266) by reduction of the dioxide obtained 127.6; Marckwald, by determining the ratio of telluric acid to tellurium dioxide, obtained 126.85; H. B. Baker (Jour. Chem. Soc., 1907, 91, p. 1849), by determining the ratio of tellurium dioxide to oxygen and by analysis of tellurium tetrabromide, obtained 127.60, and V. Lenher (Jour. Amer. Chem. Soc., 1909, 31, p. 20), by heating the double salt, TeBr4.2KBr, first in chlorine and finally in a current of hydrochloric acid to convert it into potassium chloride, obtained the value 127.55. P. E. Browning and W. R. Flint (Amer. J. Sci., 1909 (iv.), 28, p. 347) claim to have separated two substances (of atomic weights 126.49 and 128.85 respectively) from tellurium, by fractional precipitation of tellurium chloride with water, but in the opinion of H. B. Baker this would seem to point to the fact that the tellurium used was insufficiently purified, since his work showed that there was no difference between the first and last fractions (see Chem. Soc. Ann. Rep., 1909, 6, p. 39). Marckwald (Ber., 1903, 36, p. 2662) showed that the Joachimsthal pitchblende yields tellurium and a minute quantity of the strongly radioactive polonium which is precipitated by bismuth (see Radioactivity).


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Simple English

File:Tellurium
A very pure tellurium crystal

Tellurium is a chemical element. It has the chemical symbol Te. It has the atomic number 52. It has 52 protons and 52 electrons. Its mass number is 127.6. It has 8 natural isotopes. 4 are stable and 4 are radioactive. One of the radioactive ones lasts longer than any other isotope. It has a half life of 2.2 x 1024 years (2,200,000,000,000,000,000,000,000 years).

Contents

Properties

Physical properties

File:Tellurium'.jpg
Tellurium in a tube

It is a brittle silver-white semimetal. When it is pure it has a metallic shine. It is ground easily. It can be made in an amorphous form. It is a semiconductor. It changes conductivity a little when light shines on it, similar to selenium. It is corrosive to many metals when molten.

Chemical properties

Tellurium is an unreactive element. It can react with reactive metals to make tellurides. It can burn in air to make tellurium dioxide. It can be oxidized even more to tellurium trioxide. It does not corrode. The chemistry of tellurium is similar to some chemistry of selenium and sulfur, although its compounds are more reactive and the element is less reactive. It does not dissolve in most acids, although it dissolves in concentrated sulfuric acid to make a special red tellurium cation.

Chemical compounds

Tellurium makes chemical compounds in several oxidation states: -2, +2, +4, and +6. -2 compounds are normally found in tellurides. They are strong reducing agents. Tellurides are normally the main ore of tellurium. +2 compounds are found in some tellurium halides, like tellurium(II) chloride and tellurium(II) bromide. +4 compounds are found in tellurites and tellurous acid. They are weak oxidizing agents, that can be reduced to tellurium. +6 compounds are found in tellurates and telluric acid. They are powerful oxidizing agents.

Occurrence

[[File:|thumb|Tellurium as an element in quartz]] [[File:|thumb|Tellurium as an element in the ground]] Tellurium is a very rare mineral. There is 14 times more silver in the earth than there is tellurium. Tellurium is sometimes found as an element, but most of the times is found as tellurides. Gold tellurides are found in the earth. They are valuable ores of both tellurium and gold. This gold ore was not recognized as gold during one gold rush and was used as a filler. It was then discovered that it was gold telluride, making another gold rush. Telluride cannot replace sulfide in elements like selenide does.

Preparation

Tellurium can be taken from gold telluride by dissolving the gold telluride in concentrated sulfuric acid. The tellurium dissolves to make a red solution, while the gold sinks to the bottom.

A more common way of extracting tellurium from tellurides is to heat the tellurides. The tellurides are heated with sodium carbonate and air. This makes sodium tellurite. Selenites are normally found as an impurity. They are separated by reacting them with sulfuric acid. The selenites stay in solution. The tellurites turn into tellurium dioxide. Then the tellurium dioxide is reacted with sulfur dioxide dissolved in sulfuric acid to make tellurium metal. The tellurium can be melted and reformed to make bars of tellurium metal.

Uses

The main use of tellurium is in alloys. It is used in iron, copper, and lead alloys. It makes the metals more easily machinable (able to be shaped by a machine). It improves strength and durability of lead and makes it more resistant to corrosion by sulfuric acid.

Tellurium is also used in cadmium telluride solar cells. These are very efficient. It can be alloyed with both cadmium and mercury to make mercury cadmium telluride, an infrared sensitive semiconductor. It is used in some rewritable (able to be erased and written again) optical discs. Lead telluride is used in another type of infrared sensor.

It is also used to color ceramics. It is used to make fiberglass that is used in telecommunications (telephones, internet, etc.). It helps increase the refraction. It is also used in delay blasting caps. Rubber can be vulcanized by tellurium. This makes the rubber heat resistant.

In biology

Tellurium is not really used in any living things. Some fungi, though, can use tellurium instead of selenium or sulfur. Most organisms can metabolize tellurium to make dimethyl telluride, which is a garlic-smelling chemical. If someone eats a tellurium compound, it gives them garlic breath.

Safety

Tellurium is not very toxic. It can make breath stinky. It is hard to get poisoned from it because it is so rare.

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