thallium

noun

thal·​li·​um ˈtha-lē-əm 

: a soft poisonous metallic element that resembles lead in physical properties, occurs sparsely in a number of common ores, and is used chiefly in the form of compounds especially in photosensitive devices and formerly as a pesticide see Chemical Elements Table

Examples of thallium in a Sentence

Recent Examples on the Web

In April, her gym water bottle was allegedly contaminated with cyanide and thallium after her car was broken into, prosecutors said, according to WKOW. —Samira Asma-Sadeque, People.com, 24 June 2025

The two metals appear on the same line in the periodic table with their proton count and also their atomic number separated only by mercury (80) and thallium (81). —Tim Treadgold, Forbes.com, 13 May 2025

The team was able to identify proton loss not only associated with the formation of gold, but also with the production of lead, thallium and mercury atoms. —Victoria Corless, Space.com, 13 May 2025

Read more Particle Physics 'Space Jaws': NASA Reveals Roaming Monster Black Hole That's Eating Stars By detecting the number of protons and neutrons ejected in collisions, researchers were able to distinguish between the creation of other heavy elements like thallium and mercury—and gold. —Melissa Fleur Afshar, MSNBC Newsweek, 9 May 2025

Word History

Etymology

New Latin, from Greek thallos green shoot; from the bright green line in its spectrum

First Known Use

1861, in the meaning defined above

Rhymes for thallium

• allium
• gallium
• pallium

Kids Definition

thallium

noun

thal·​li·​um ˈthal-ē-əm 

: a soft poisonous metallic element that has characteristics similar to lead see element

Medical Definition

thallium

noun

thal·​li·​um ˈthal-ē-əm 

: a sparsely but widely distributed poisonous metallic element that resembles lead in physical properties and is used chiefly in the form of compounds in photoelectric cells or as a pesticide

—symbol Tl see Chemical Elements Table