Properties and Overview of Thallium

Overview:

Thallium (Tl) represented by the chemical symbol Tl and atomic number 81, is a soft, malleable metal belonging to the periodic table's post-transition metals group. It was discovered in 1861 by Sir William Crookes while investigating the spectra of selenium compounds. The element is named after the Greek word "thallos," meaning "green shoot or twig," due to the bright green spectral line that indicates its presence. Physically, thallium is a bluish-white metal that resembles lead in appearance. It is soft enough to be cut with a knife and highly malleable, allowing it to be shaped easily. Thallium has a relatively low melting point of 304°C and a boiling point of 1,473°C. When exposed to air, thallium quickly develops a grayish oxide layer, which helps protect it from further oxidation. However, thallium can corrode rapidly in moist air, forming a blackish surface layer of thallium hydroxide.
Chemically, thallium exhibits two primary oxidation states: +1 and +3. The +1 oxidation state is more stable and is the form in which thallium most commonly exists in its compounds. Thallium(I) compounds, such as thallium chloride (TlCl) and thallium sulfate (Tl₂SO₄), are typically colorless or white and soluble in water. In contrast, thallium (III) compounds, like thallium(III) oxide (Tl₂O₃), are less stable and can be reduced to the +1 state easily. thallium is chemically similar to the alkali metals, particularly potassium, which is reflected in the similarity between their ionic radii and reactivities. However, thallium's reactivity is generally higher, forming various compounds, including oxides, halides, and sulfates.
Regarding safety, thallium is highly toxic and must be handled with extreme care. It is readily absorbed through the skin, lungs, and digestive system, making exposure to thallium and its compounds particularly dangerous. Thallium poisoning can occur from ingestion, inhalation, or prolonged skin contact, leading to severe symptoms such as hair loss, damage to peripheral nerves, and organ failure. Thallium compounds, particularly thallium sulfate, were once used as rat poisons and insecticides. However, these uses have been discontinued mainly due to the risks they pose to humans and non-target species. In the event of thallium exposure, prompt medical treatment is essential, and antidotes such as Prussian blue can help remove thallium from the body.

Production:

Thallium is typically produced as a byproduct of the smelting of zinc, lead, and copper sulfide ores. During the extraction of these metals, thallium accumulates in the flue dust and can be recovered through a series of chemical processes. The dust is treated with acid, and thallium is precipitated as thallium(I) chloride, which can be further purified. Another method of obtaining thallium involves leaching ores with sulfuric acid, followed by selective precipitation.

Applications:

Thallium has several specialized applications despite its toxicity. One of its primary uses is in the electronics industry, where thallium is used to manufacture low-melting-point glass and optical lenses. Thallium-based glasses have high refractive indices and low dispersion, making them valuable in producing specialized optical instruments. In addition, thallium is used in the production of semiconductor materials, such as thallium selenide (Tl₂Se) and thallium bromide (TlBr), which are used in infrared detectors and other electronic devices.
Thallium is also employed in the medical field, particularly in diagnostic imaging. Thallium-201, a radioactive isotope of thallium, is used in small doses in nuclear medicine, specifically in stress tests for diagnosing coronary artery disease. The isotope is administered to patients, and its distribution in the heart is monitored using a gamma camera, providing valuable information about blood flow and heart function. However, due to thallium's toxicity, its use is strictly controlled and limited to particular medical applications.
Additionally, thallium has been used historically in producing thermoelectric materials, which convert temperature differences into electrical energy. Thallium-containing materials, such as thallium telluride, have been studied for their efficiency in thermoelectric power generation, particularly in applications where heat recovery is critical. However, the use of thallium in such applications is declining due to concerns about its environmental and health impacts.

Summary:

Thallium is a highly toxic, bluish-white metal with significant chemical reactivity, existing primarily in the +1 oxidation state. It is produced as a byproduct of zinc, lead, and copper smelting and is used in various specialized applications, including electronics, optical materials, and medical diagnostics. The toxicity of thallium and its compounds necessitates careful handling and has led to the decline of its use in many applications, particularly those that pose a risk of exposure to humans or the environment.

See a comprehensive list of atomic, electrical, mechanical, physical and thermal properties for thallium below: