- Thallium (Tl) is a post-transition metal with atomic number 81, belonging to Group 13 of the periodic table, alongside boron, aluminum, gallium, and indium.
- Its electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹, giving it three valence electrons, though it most often exhibits +1 and +3 oxidation states. Each atom of thallium contains eighty-one protons, eighty-one electrons, and typically around 123 neutrons depending on the isotope.
- Naturally occurring thallium consists of two stable isotopes: thallium-203 (²⁰³Tl) and thallium-205 (²⁰⁵Tl), with the latter being more abundant.
- Thallium was discovered in 1861 by the English chemist Sir William Crookes, who identified it through its bright green spectral emission lines while analyzing residues from sulfuric acid production. The element’s name comes from the Greek word thallos, meaning “green shoot” or “twig,” in reference to this characteristic green spectral line. French chemist Claude-Auguste Lamy independently isolated thallium around the same time, further confirming its discovery.
- Physically, thallium is a soft, malleable, and bluish-gray metal resembling lead. It has a density of 11.85 g/cm³, a melting point of 304 °C (579 °F), and a boiling point of 1,473 °C (2,683 °F). Thallium is so soft it can be cut with a knife and tarnishes quickly in air, forming a bluish oxide layer. When finely divided, thallium is highly reactive and can even ignite spontaneously.
- Chemically, thallium shows similarities to alkali metals in its +1 oxidation state, making it biologically dangerous due to its ability to mimic potassium in living systems. In the +3 state, it resembles other Group 13 elements. Thallium compounds include thallium(I) sulfate (Tl₂SO₄), once used as a rodenticide and insecticide, and thallium(III) oxide (Tl₂O₃), used in optical materials. However, due to extreme toxicity, most of these uses have been banned or severely restricted.
- Thallium has niche applications in modern technology. Certain thallium compounds are used in optics and electronics, particularly in infrared detectors, high-refractive index glass, and photoelectric cells. Thallium bromide-iodide crystals (TlBrI) are used in infrared optics, while thallium-based superconductors have been investigated for advanced materials. Radioactive isotopes of thallium, such as thallium-201 (²⁰¹Tl), are used in nuclear medicine for diagnostic imaging, especially in detecting heart disease through thallium stress tests.
- Biologically, thallium is extremely toxic, with no known beneficial role in living organisms. Thallium salts are readily absorbed through the skin, respiratory tract, or digestive system, making exposure highly dangerous. Poisoning symptoms include hair loss, nerve damage, and organ failure. Because it mimics potassium, thallium disrupts essential cellular processes, leading to widespread toxicity. Historically, thallium poisoning has been implicated in accidental exposures, criminal cases, and even political assassinations.
- Environmentally, thallium is a rare element in Earth’s crust, with an abundance of about 0.7 parts per million. It is typically obtained as a by-product of smelting sulfide ores of lead, zinc, and copper. Major producers include China, Kazakhstan, and Russia. Industrial pollution, coal combustion, and mining activities are the main sources of thallium contamination in soil and water. Due to its toxicity, thallium is considered a significant environmental hazard, and monitoring is crucial to protect ecosystems and human health.
