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(thăl`ēəm), metallic chemical element; symbol Tl; at. no. 81; interval in which at. wt. ranges 204.382–204.385; m.p. 303.5°C;; b.p. about 1,457°C;; sp. gr. 11.85 at 20°C;; valence +1 or +3. Thallium is a soft, malleable, lustrous silver-gray metal with a hexagonal close-packed crystalline structure. A member of Group 13 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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, it resembles aluminum in its chemical properties. In its physical properties it resembles lead. It forms univalent compounds similar to those of the alkali metals. It tarnishes rapidly in dry air, forming a heavy oxide coating; in moist air or water the hydroxide is formed. It dissolves in nitric or sulfuric acid. Thallium is widely distributed in nature, but the only minerals rich in the element are crooksite and lorandite. It is also found in copper pyrites and lead and zinc ores; it is recovered during the processing of these ores, the method of recovery depending on the source. Thallium is used in low-melting alloys with other metals and in compounds. Both the metal and its compounds are very poisonous. The sulfide is used as a rat poison and the sulfate as an insecticide. The oxide is used in special highly refractive optical glass. Several compounds are used in photoelectric cells and infrared detectors. Discovered spectroscopically in 1861 by Sir William Crookes, it was isolated independently by Crookes and C. A. Lamy in 1862.



Tl, a chemical element of group III in Mendeleev’s periodic system. Atomic number, 81; atomic weight, 204.37.

Thallum has a gray luster when freshly cut. It belongs to the class of rare dispersed elements. In nature the element occurs in the form of two stable isotopes, 203T1 (29.5 percent) and 205T1 (70.5 percent), and in the form of the radioactive isotopes 207T1–210T1, which are members of radioactive series. The isotopes 202T1 (half-life T½ = 12.5 days), 204T1(T½ = 4.26 years), and 206T1 (T½= 4.19 min) have been produced artificially.

Thallium was discovered in 1861 by W. Crookes through spectroscopic analysis of slurry from sulfuric acid production; Crookes noticed the element’s characteristic green line in the spectrum. (Its name comes from the Greek thallos, “young, green branch.”) In 1862 the French chemist C. A. Lamy isolated thallium and determined its metallic nature.

Occurrence. The average thallium content in the earth’s crust (clarke) is 4.5 × 10–5 percent by weight, but because of the metal’s extreme dispersion, it plays an insignificant role in natural processes. In nature it occurs primarily in monovalent and, less frequently, trivalent compounds. Like the alkali metals, thallium is concentrated in the upper part of the earth’s crust, the granite layer, where its average content is 1.5 × 10–4 percent. Smaller quantities (2 × 10–5 percent) are found in basic rocks, and the content in ultrabasic rocks is only 1 × 10–6 percent. Only seven thallium minerals, including crookesite, lorandite, and vrbaite, are known, and all are extremely rare. Thallium displays the most geochemical similarity to K, Rb, and Cs, as well as to Pb, Ag, Cu, and Bi. (SeeDISPERSED ELEMENT and DISPERSED-ELEMENT ORE.) It migrates readily in the biosphere and is sorbed from natural waters by coals, clays, and manganese hydroxides. Thallium accumulation occurs upon evaporation of water—for example, up to 5 × 10–8 grams per liter in Lake Sivash.

Physical and chemical properties. Thallium is a soft metal that oxidizes readily and dulls rapidly upon exposure to air. At a pressure of 0.1 meganewton per sq m (MN/m2), or 1 kilogram-force per sq cm (kgf/cm2), and a temperature below 233°C, thallium has a close-packed hexagonal lattice with a = 3.4496 angstroms (Å) and c = 5.5137 Å. Above 233°C it has a body-centered cubic lattice with a = 4.841 Å, and at an elevated pressure of 3.9 GN/m2, or 39,000 kgf/cm2, a face-centered cubic lattice. It has a density of 11.85 g/cm3, an atomic radius of 1.71 Å, and ionic radii of 1.49 Å for T1 and 1.05 Å for Tl3+. Its melting point is 303.6°C, and its boiling point, 1457°C. It has a specific heat capacity of 0.13 kilojoule/kg·K (0.031 calorie/g·C) at 20°–100°C. Its coefficient of linear thermal expansion is 28 × 10–6 at 20°C and 41.5 × 10–6 at 240°–280°C, and its thermal conductivity is 38.94 watts per (m·°K), or 0.093 calorie per (cm·sec·°C). Its electrical resistivity at 0”C is 18 × 10–6ohm‧cm, and its thermal coefficient of electrical resistivity is 5.177 × 10–3 to 3.98 × 10–3 at 0°–100°C. The temperature of transition into the superconductive state is 2.39°K. Thallium is diamagnetic, with a mass susceptibility of –0.249 × 10–6 at 30°C.

The outer electron shell of the Tl atom has the configuration 6s26p1; Tl compounds have oxidation states +1, for T1(I), and + 3, for Tl(III). Thallium reacts with oxygen and halides even at room temperature and with sulfur and phosphorus upon heating. It dissolves freely in nitric acid and less readily in sulfuric acid; it is insoluble in hydrogen halides and formic, oxalic, and acetic acids. It does not react with solutions of alkalies. Freshly distilled water containing no free oxygen does not act on thallium. Thallium’s main compounds with oxygen are thallous oxide, T12O, and thallic oxide, T12O3.

Thallous oxide and salts of T1(I) nitrate, sulfate, and carbonate are soluble. Thallium chromate, bichromate, halides (except the fluoride), and oxide are sparingly soluble in water. Tl(III) forms many complexes with inorganic and organic ligands. The halides of Tl(III) are freely soluble in water. T1(I) compounds are of the greatest practical value.

Preparation and use. Commercial thallium is obtained industrially as a by-product during the processing of sulfide ores of non-ferrous metals and iron. It is extracted from the intermediates of lead, zinc, and copper manufacture. The selection of the method for processing the raw material depends on the material’s composition. For example, thallium and other valuable components are extracted from the dust obtained in lead manufacture by fluidized-bed sulfatization of the material at 300°–350°C. The resultant sulfate mass is leached with water, and the thallium is extracted from the solution using an iodine-containing 50 percent tributyl phosphate solution in kerosine, then reextracted using sulfuric acid (300 g per liter), with the addition of 3 percent hydrogen peroxide. The metal is isolated from the reextracts by roasting on zinc sheets. Subsequent smelting under a layer of sodium hydroxide yields 99.99 percent pure thallium. Electrolytic refining and recrystallization are used to produce a metal of higher purity.

In technology, thallium is mainly used in compound form. Single crystals of solid solutions of the halides TIBr-TlI and T1C1-TlBr, known in technology as KRS-5 and KRS-6, are used in the manufacture of optical components for infrared devices. Crystals of T1C1 and TICl-TlBr serve as radiators in Cherenkov counters. Tl2O is a constituent of certain types of optical glass. Sulfides, hydroxysulfides, selenides, and tellurides of thallium are components of semiconductor materials used in the manufacture of photoconductive cells, semiconductor rectifiers, and vidicons. An aqueous solution of a mixture of thallous formate and thallous malonate, called Clerici solution, is widely used to separate minerals by density. Thallium amalgam, which solidifies at – 59°C, is used in low-temperature thermometers. Metallic thallium is used in the preparation of bearing alloys and fusible alloys, and in oxygen meters to determine oxygen concentration in water. 204T1 is used in radioisotope devices as a source of β-emission.


Thallium in the organism. Thallium is always present in plant and animal tissue. Its average content in the soil is 10–5 percent, in seawater 10–9 percent, and in animals 4 × 10–5 percent. In mammals, thallium is readily absorbed from the gastrointestinal tract, accumulating mainly in the spleen and muscles. In humans, the daily thallium intake with food and water is about 1.6 micrograms (μ.g), and from the air, 0.05 μg. The biological role of thallium in the body is still unclear. Thallium is moderately toxic to plants and highly toxic to humans and other mammals.

Poisoning by thallium and its compounds is possible during preparation and use. Thallium enters the body through the respiratory organs, intact skin, and the digestive tract; it is eliminated over a long period of time, primarily in urine and feces. Acute, subacute and chronic poisonings have similar clinical pictures, differing in the clarity and rate of appearance of symptoms. In acute cases, disorders of the gastrointestinal tract (nausea, vomiting, stomach pains, diarrhea, constipation) and the respiratory tract appear after one or two days. Hair loss and signs of avitaminosis, such as smoothing of the mucous membrane of the tongue and cracks in the corners of the mouth, appear after two to three weeks. Severe cases may involve the development of polyneuritis, mental disorders, impairment of vision, and other symptoms. Preventive measures for occupational poisoning include mechanization of production processes, hermetic sealing of equipment, ventilation, and the use of protective clothing.



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A metallic element in group 13, symbol Tl, atomic number 81, atomic weight 204.383; insoluble in water, soluble in nitric and sulfuric acids, melts at 302°C, boils at 1457°C.
Bluish-white metal with tinlike malleability, but a little softer; used in alloys.


a soft malleable highly toxic white metallic element used as a rodent and insect poison and in low-melting glass. Its compounds are used as infrared detectors and in photoelectric cells. Symbol: Tl; atomic no.: 81; atomic wt.: 204.3833; valency: 1 or 3; relative density: 11.85; melting pt.: 304°C; boiling pt.: 1473±10°C