Element 81, also known as thallium, is a relatively rare, moderately abundant and very toxic metal. It is a post-transition metal, which means that it has three valence electrons. It is very soft and can be easily melted, making it highly flammable.
It was discovered spectroscopically in 1861 by Sir William Crookes. He was examining residues from the sulfuric acid plant in London when he detected a green line that was not present in other elements he had observed previously.
The name thallium comes from the Greek word "thallos", meaning a 'green twig', and was chosen by Crookes as an appropriate name for the new element. His findings were published in Chemical News in March 1861.
This chemical element can be found in a number of minerals, including some that contain the heavy-metal sulfides silver, copper, lead, zinc and antimony. It is also extracted from some manganese nodules on the ocean floor.
Several commercial uses are also possible for this element, including low-temperature thermometers, photoresistors and infrared optical equipment. Thallium amalgam is used in these products, since it freezes at -58 degC, whereas pure mercury does not.
Soluble thallium salts are very toxic and can be found in the soil and water. They were historically used as rat poisons and insecticides, but they have been discontinued in many countries. These are nonselective toxins, so they disrupt many biological processes and can be fatal. This is the reason why they are banned in some countries.
