We continue to review some of the most important materials in heat treatment and metallurgy.  

Thallium (chemical symbol: Tl)

Thallium (Fig. 1) is a malleable and lustrous silvery metal that resembles lead in physical appearance. It has a low melting point and will tarnish in air to a bluish-gray oxide, and it is so soft that it can be easily cut with a knife. Both elemental thallium and its compounds are highly toxic, and direct contact with the skin should be avoided. In fact, thallium sulfate (Fig. 2) was the poisoning agent in the Agatha Christie novel The Pale Horse in 1961, where she accurate describes the symptoms of thallium poisoning: lethargy, numbness, blackouts, slurred speech and general debility.

Sir William Crookes first discovered thallium in 1861 when he was extracting selenium from a deposit leftover in a sulfuric-acid factory. At first the leftover residue after the selenium extraction appeared to be the element tellurium, but he was unsuccessful at isolating it. He analyzed the residue spectroscopically and was unable to match the bright green line in the spectrum with any known element.

Further analysis determined that what he discovered was the presence of a new element. He named it after the Greek word “thallos,” meaning a green shoot or twig referring to the bright green spectral emission line he observed. Crookes was unable to isolate the pure element due to having such a limited supply. However, it was isolated only a year later by Claude-Auguste Lamy, an independent researcher who had a larger supply to work with.

Thallium has applications that span multiple industries and has a long history of use. The tasteless and odorless thallium sulfate was once marketed as a powerful rat poison and ant killer, but it was prohibited from being sold in many countries, including the U.S., since 1972 for safety reasons. Thallium is also seen in photoresistors (Fig. 3) because of its electrical-conductivity changes when exposed to infrared light. Furthermore, it can also be used to make glass that has a high index of refraction for specialty purposes.

Thallium iodide is also used as an additive in metal-halide lamps, where it helps optimize lamp temperature and color rending to shift the spectral output of the light to the green region, which makes it ideal for underwater lighting applications.

Here are a few important facts about thallium.[2,5]

  • Atomic number: 81
  • Atomic weight: 204.3833
  • Melting point: 577 K (304°C or 579°F)
  • Boiling point: 1746 K (1473°C or 2683°F)
  • Density: 11.8 grams per cubic centimeter
  • Phase at room temperature: Solid
  • Element classification: Metal
  • Period number: 6    
  • Group number: 13    
  • Group name: none
  • Electron configuration: [Xe] 4f14 5d10 6s2 6p1



  1. KnowledgeDoor (www.knowledgedoor.com)
  2. Jefferson Lab (https://www.jlab.org)
  3. Chemicool (www.chemicool.com/)
  4. Wikipedia (www.wikipedia.org)
  5. University of Bristol (www.chm.bris.ac.uk/)