Thallium is a chemical element with symbol Tl and atomic number 81. This soft gray post-transition metal is not found free in nature. When isolated, it resembles tin, but discolors when exposed to air.
Three reasons Thallium stands out
1
Once used in rat poisons, now has uses in medical imaging.
2
It is extremely insidious as a poison because it is completely colourless, odourless, and tasteless in solution.
3
Its distinctive symptom of causing sudden hair loss has helped forensic investigators solve several famous poisoning cases.
The poison once mistaken for potassium
Thallium ions can imitate potassium closely enough to enter biological pathways, making many compounds dangerously toxic. Its salts were once sold as rat poison before restrictions tightened. Today specialized thallium isotopes aid heart imaging, and thallium-containing crystals detect infrared light and radiation.
Recognizing Thallium in Elementle
If Thallium is the mystery element, guesses below atomic number 81 point up and guesses above it point down. Its symbol is Tl: a green letter matches the same position in that symbol, while a yellow letter belongs in another position.
The family clue turns green for metal guesses. Compare Lead (Pb), number 82 and Bismuth (Bi), number 83: they share the family clue, but their atomic numbers and symbols help you tell them apart.
Values are reference data, not a specification for every temperature, pressure, or allotrope. Unknown properties are left unavailable. Superheavy-element predictions remain uncertain.
The introduction and original editorial story are adapted from Wikipedia contributors via Periodic-Table-JSON. Element descriptions and their adaptations are available under CC BY-SA 3.0. Numerical attributes use the PubChem periodic-table dataset, retrieved September 22, 2026.