thallium

 
Pronunciation: /ˈθalɪəm/

noun

[mass noun]
  • the chemical element of atomic number 81, a soft silvery-white metal which occurs naturally in small amounts in iron pyrites, sphalerite, and other ores. Its compounds are very poisonous. (Symbol: Tl)

Origin:

mid 19th century: modern Latin, from Greek thallos 'green shoot', because of the green line in its spectrum