Element 81 · Post-Transition Metal
Thallium
Thallium is so toxic it was once called "the poisoner's poison" — its limited applications keep it strictly within specialist industry.
Element Facts
SYMBOL
Tl
ATOMIC NO.
81
ATOMIC WEIGHT
204.38
CATEGORY
Post-Transition Metal
PERIOD
Period 6
GROUP
Group 13
Discovery & History
Thallium was discovered in 1861 by British scientist William Crookes while examining the residues left over from sulfuric acid production using pyrite. Using spectroscopy, he observed a brilliant green spectral line that matched no known element. He named it after the Greek thallos, meaning "green shoot" or "green twig," for this distinctive colour. Shortly after, French chemist Claude Auguste Lamy independently isolated thallium metal and characterised its properties.
In the 19th and early 20th centuries, thallium found use as a rodent and insect poison — thallium sulfate was widely used in rat poison and ant killer, sold under various commercial names. It was effective but indiscriminate: its tasteless, odourless nature made it also the poison of choice in numerous celebrated murder cases, earning it the nickname "the poisoner's poison."
Most countries banned thallium-based pesticides in the 1970s and 1980s due to their extreme toxicity to humans and wildlife. Modern uses are limited and highly specialised: thallium-doped sodium iodide crystals are used as gamma-ray detectors in medical imaging and physics research, and thallium compounds have niche applications in infrared optics.
Quick Overview
Thallium is element 81, a soft, malleable post-transition metal discovered in 1861 by Sir William Crookes. It was famously used as a homicidal poison in the early 20th century because it is colourless, tasteless, and acts slowly — Agatha Christie used it as the murder weapon in her novel "The Pale Horse."
Where It's Used
Specialist optical glass for high-refractive-index lenses. Some semiconductor applications. Pharmaceutical and medical imaging compounds (thallium-201 isotope for cardiac stress test imaging).
Can You Sell It?
Why QuickStop Metals doesn’t buy Thallium:
Thallium and its compounds are extremely toxic — fatal in small doses. Applications are restricted to specialist industries with appropriate handling procedures. There is no general scrap market for thallium.
Price Guide
Thallium is a chemical commodity sold in small quantities to specialist users.