thallium

noun

Etymology: Coined based on Ancient Greek θαλλός (thallós, “green branch”) (after the color of the radiation spectra), + -ium.

1. A metallic chemical element (symbol Tl) with atomic number 81: a gray post-transition metal that discolors when exposed to air.

   “Environmental authorities in Lüeyang County, Shaanxi province, discovered excessive levels of thallium in two of the Jialing River’s upstream tributaries on Jan. 20, according to a statement Monday. The concentration of thallium that day was 170% higher than the standard for Qingni River, one of the upstream tributaries bordering Gansu province.”

2. A single atom of this element.