Thallium (Tl)
Thallium (Tl) is a chemical element with the atomic number 81, situated in Group IIIA of the periodic table. Resembling lead, thallium is a soft, dense, shiny metal that tarnishes to a blue-gray color when exposed to air. Although it is widely distributed in the Earth's crust, thallium is relatively rare and primarily obtained as a by-product from the extraction of lead and zinc ores. Its primary applications are in specialized sectors, including electronics, glassmaking, and infrared optical systems. Historically, thallium was used in pesticides and as a rat poison, but these uses have diminished due to its toxicity. Thallium compounds can be hazardous, with exposure leading to symptoms such as hair loss and nervous system disorders. Today, thallium is utilized in limited, specific applications, such as high-temperature superconductors and scintillation detectors for gamma rays, while its toxic nature necessitates careful handling.
Thallium (Tl)
Where Found
Thallium is widely distributed in the Earth’s crust in small amounts. A few minerals exist that contain up to 60 percent thallium, but these are extremely rare. The most important sources of thallium are zinc and lead ores.
Primary Uses
Thallium compounds are used in very small amounts for special applications in electronics and glassmaking. Thallium was used previously in pesticides.
Technical Definition
Thallium (abbreviated Tl), atomic number 81, belongs to Group IIIA of the periodic table of the elements and resembles lead in its chemical and physical properties. It has two naturally occurring isotopes and an average atomic weight of 204.37. Pure thallium is a soft, dense, shiny metal that dulls to a blue-gray tinge when exposed to air. Its density is 11.85 grams per cubic centimeter; it has a melting point of 303.5° Celsius and a boiling point of 1,457° Celsius.
Description, Distribution, and Forms
Thallium is a fairly rare element resembling lead. It is mostly obtained as a by-product of the extraction of lead or zinc from sulfide ores or from copper smelting. Most of this production takes place in the United States (recovery from flue dust and smelters), Canada, and Europe. Once used in pesticides, thallium is now used to a limited extent in manufacturing photoelectric devices and in making special kinds of optical equipment.
History
Thallium was discovered in 1861 by the British chemist and physicist Sir William Crookes. Its first important industrial use was as a rat poison and insecticide in the form of thallium sulfate, first used in Germany in the 1920’s. In the 1960’s, thallium compounds fell out of use for this purpose.
Obtaining Thallium
Thallium is usually obtained from the sulfide ores of zinc and lead. When these ores are heated to extract the zinc or lead, dust and gas that contain thallium compounds, as well as compounds of elements such as cadmium, indium, selenium, and tellurium, are released. Thallium compounds are separated from the other compounds by a variety of chemical processes. In general, these methods involve forming compounds of thallium that have higher or lower solubilities in certain liquids than the equivalent compounds of the other elements. crystallization removes the least soluble compound.
Free thallium may be obtained from the compound by electrolysis, resulting in a powder. It may then be transformed into metallic form by compressing it, heating it in the absence of oxygen, and casting it into molds.
Uses of Thallium
Thallium is not a major resource in manufacturing, but it has a few special uses. Thallium sulfide may be used to make photoelectric cells that are highly sensitive to infrared light. Thallium bromide and thallium iodide can be used to produce crystals that transmit infrared light. These crystals may then be used to make lenses, windows, and prisms for use in infrared optical systems. Thallium oxide may be used to make special kinds of glass or to add color to artificial gems.
Thallium-barium-calcium-copper oxide high-temperature superconductors are used in wireless communication devices. Sodium iodide crystals doped with thallium are used in scintillometers for the detection of gamma rays. Thallium increases the refractive index and density of glass; it is employed as a catalyst for the synthesis of organic compounds; it is used in high-density liquids that are employed in mineral-separation methods; it is also alloyed with mercury to measure low temperature. Finally, thallium 201, a radioactive isotope, is used in cardiovascular imaging.
Thallium compounds are often toxic, as shown by their former use in pesticides. Thallium poisoning is rare but can be fatal. Symptoms of thallium toxicity include rapid hair loss and disorders of the digestive and nervous systems.
Bibliography
WebElements.