Tl - Thallium
|Chemical Element||Thallium||Melting Point °C||304|
|Chemical Symbol||Tl||Boiling Point °C||1457|
|Atomic Number||81||Density g/cm3||11.9|
|Atomic Weight||204.383||Oxide||Tl2O and Tl2O3|
Thallium is a soft, silvery-white metal, which is very heavy and resembles lead in appearance. Like gold, it is soft enough to be cut with a knife. It belongs to Group 13 of the Periodic Table and is the 59th most abundant element on Earth. Thallium tarnishes readily in moist air, forming a heavy grey oxide, which will eventually flake off to expose a fresh layer that will itself tarnish. It reacts with steam to produce Thallium hydroxide (TlOH). The metal is attacked by acids, nitric acid being the most rapid. Thallium is widely dispersed, and ten times more abundant than silver.
Two men are credited with the discovery of thallium: William Crookes and Claude Auguste Lamy. Whilst observing the atomic spectrum of a sample of impure sulphuric acid, Crookes observed a brilliant green line with a wavelength unlike any other previously discovered element. This meant that he had come across a new element. This line is responsible for the name chosen for the element, “Thallos” being the Greek word for ‘green twig or shoot’, and therefore Crookes is the man who first found and named the element. Crookes published his discovery in the March 1861 issue of Chemical News and went about trying to obtain a pure sample, which eluded him. Meanwhile, in 1862, Lamy began to research thallium in greater detail and was able to produce a pure sample of the new element. The French Academy therefore credited him with the discovery and sent the sample off to the London International Exhibition, where it was recognised as a new metal, and Lamy was awarded a medal. Crookes became aware of this and demanded he be credited with the discovery. The exhibition committee then felt obliged to also award him a medal, and the discovery is now credited to both scientists.
Thallium is found in low concentrations, for example as the sulphide ores of heavy metals such as zinc and lead, and as the mineral of caesium, potassium and rubidium. In 2005 a large thallium-only deposit was discovered in Xiangquan, and in 2006 a thallium-rich murunskite deposit was discovered on the Russian Kola Peninsula. Other important thallium mineral deposits include Norilsk-Talnakh deposit in Siberia and another at Allchar in Macedonia. There has been no assessment of how large reserves are. Commercially, thallium is recovered from flue dusts and residues following heavy metal processing, but can also be found together with sulphides of antimony, arsenic and silver. Occasionally it can be found as a by-product of sulphuric acid production. The world’s largest producers are Kazakhstan and China, with Kazzinc in Kazakhstan being the world’s largest producer.
The two largest consumers of thallium are the fibre optics industry, and the production of glass lenses. Due to these two main applications, China, Japan and South Korea are the main consumers. There are also other applications for the metal. Owing to the low melting point of thallium, it is particularly useful in combination with mercury in thermometers, which are then able to record temperatures 20°C lower than mercury-only thermometers, although it is not usually used as an alloying agent. Although thallium sulphate is banned in Western countries, it is still sold in some developing countries for use as a pesticide.
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- Gray, Theodore. The Elements, A Visual Exploration of Every Known Atom in the Universe, Black Dog & Leventhal Publishers, Inc, NY, 2009
- Stwertka, Albert. A Guide to the Elements, 3rd Edition, Oxford University Press, 2012