Chemical Element: thallium

(Modern Latin: from Greek, thallos, "a young, or green, twig or shoot" [based on the color of its spectrum]; metal)

Chemical-Element Information

Symbol: Tl
Atomic number: 81
Year discovered: 1861

Discovered by: Sir William Crookes (1832-1919), a British physicist.


  • Thallium was discovered spectroscopically, in 1861, by Crookes as he examined the flue dust produced in the roasting of seleniferous pyrites.
  • The element was named after the green spectral line, that identified it and was foreign to all of the then known spectra and so he concluded that the mineral contained a new element, which he named thallium, from Greek for “a green twig”.
  • Crookes presumed that his thallium was something of the order of sulfur, selenium, or tellurium; but the French chemist Claude-Auguste Lamy, who anticipated him in isolating the new element (1862), found it to be a metal.
  • They had expected to isolate tellurium after removing selenium from the by-products of a commercial sulfuric acid factory, but instead found the new element thallium.
  • The pure metal is not commercially important, but thallium salts have a number of laboratory and industrial uses.
  • Thallium forms two series of salts: thallous, in which the metal is univalent; and thallic, in which it is trivalent.
  • All thallium salts are poisonous, producing symptoms somewhat resembling those of lead poisoning.

Name in other languages:

French: thallium

German: Thallium

Italian: tallio

Spanish: talio


Information about other elements may be seen at this Chemical Elements List.

A special unit about words that include chemo-, chem- may be seen here.