BOILING POINT: 1,457°C
DENSITY: 11.8 g/cm 3
MOST COMMON IONS: Tl + , Tl 3+
Thallium was discovered in 1861 by the British chemist Sir William Crookes. While attempting a spectroscopic analysis of materials that contained tellurium, Crookes observed a previously unknown bright green line
on his spectroscope (a machine that identifies the kinds of light emitted by elements at high temperatures). The name thallium comes from the Greek word Thallos, meaning "green twig," and was chosen by Crookes because the spectral line he had observed reminded him of a fresh green shoot.
The abundance of thallium in Earth's crust is estimated to be between 0.1 and 1 mg/kg (ppm). It is widely dispersed and is often associated with potassium minerals. Small deposits of the thallium-bearing minerals such as lorandite and the aptly named crookesite exist in Greece and Sweden, respectively.
Thallium metal is so soft that it can easily be cut with a knife. It has a metallic luster that slowly tarnishes upon exposure to air to give the metal a bluish-gray appearance (caused by the formation of Tl 2 O 3 ). The thin layers of surface oxide prevent further oxidation . However, in moist air or water, the soluble hydroxide TlOH is formed. Thallium can exist in two oxidation states, (I) and (III). When heated in air, the metal oxidizes to Tl 2 O. Thallium reacts vigorously with the halogens , forming dihalides of composition 2Tl X 3 where X = fluorine, chlorine, or bromine. Thallium is rapidly dissolved in nitric acid but rendered passive in sulfuric and hydrochloric acids due to the formation of insoluble Tl(I) salts.
In biological systems, thallium is nonessential, (i.e., not required for organisms to complete their life cycles) and toxic at high concentrations. Thallium(I) mimics potassium. In humans it affects potassium-activated enzymes in the brain, muscles, and skin. Symptoms of thallium poisoning can easily be attributed to other causes, which in the past has made this element popular for homicides!
Currently, thallium is used in some electronic devices, in low melting point glass, and in the creation of low melting point alloys .
Cotton, F. Albert; Wilkinson, Geoffrey; Mruillo, Carlos A.; et al. (1999). Advanced Inorganic Chemistry, 6th edition. New York: Wiley.
Emsley, John (2001). Nature's Building Blocks: An A-Z Guide to the Elements. New York: Oxford University Press.