What is Thallium?

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  • Written By: Mary McMahon
  • Edited By: Bronwyn Harris
  • Last Modified Date: 14 October 2019
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Thallium is a highly toxic metallic chemical element which is classified among the poor metals. Despite its toxicity, thallium has a number of commercial and industrial uses, although consumers do not interact with it directly. The element is most commonly extracted from the minerals lorandite and crookesite, and it can also be processed to yield useful isotopes. The antidote to thallium ingestion, by the way, is Prussian blue.

In a pure form, thallium is silvery white and extremely soft; it can easily be cut with a knife. When exposed to air, the element rapidly tarnishes, turning dull gray to black. The physical properties of thallium resemble those of lead, another element in the poor metals group. On the periodic table of elements, thallium is identified with the symbol Tl, and it has an atomic number of 81.

Credit for the discovery of thallium is typically given to Sir William Crookes, who observed its distinctive spectroscopic signature in 1861 and later succeeded in isolating the element. Spectroscopically, the element emits a characteristic green line, leading Crookes to name it after the Greek thallos, for “twig or new growth.” The element is extracted from the minerals it appears in through a smelting process.


Historically, thallium was used as an insect and rat poison, because of its toxicity. However, incidents of illness as a result of human exposure led to a discontinuance of this use of the element. It is still used in photocells, semiconductors, infrared detectors, and low melting, high density glass. Isotopes of thallium are used in nuclear medicine as contrast agents for medical imaging. It has also historically been used as a poison, because it takes only a small amount to kill someone.

This element is extremely toxic, and therefore, it should be handled with care. When the metal is being melted or cut, protection should be worn, and people should avoid skin contact with thallium, as it can be absorbed into the body. Products which contain thallium are generally clearly labeled, and any warning directions should be carefully followed. If thallium exposure is suspected, people should seek medical treatment immediately.


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