Atomic number 81 · Post-transition metal
Thallium Tl
Thallium (Tl), element 81, is a soft, grey-white metal that can be cut with a knife. Many of its compounds are soluble, tasteless and odourless, and so poisonous that they were once used as rat poison. On the other hand, radioactive thallium-201 has helped doctors image blood flow to the heart.
Key properties
| Atomic number | 81 |
|---|---|
| Atomic weight | 204.38 (CIAAW 2024 abridged standard atomic weight) |
| Category | Post-transition metal |
| Group | 13 |
| Period | 6 |
| Block | p |
| State (25 °C, 1 atm) | Solid |
| Melting point | 577 K (303.9 °C) |
| Boiling point | 1746 K (1472.8 °C) |
| Density | 11.8 g/cm³ |
| Electronegativity (Pauling) | 1.62 |
| Atomic radius (van der Waals) | 196 pm |
| First ionization energy | 6.108 eV |
| Electron affinity | 0.2 eV |
| Oxidation states | +3, +1 |
| Discovered | 1861 |
Electron configuration
- Condensed
- [Xe]6s2 4f14 5d10 6p1
- Full
- 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1
- Electrons per shell
- 2 · 8 · 18 · 32 · 18 · 3
Orbital diagram
Position in the table
Discovery
In 1861 William Crookes in England was examining the flame spectrum of selenium-bearing residues from a sulfuric-acid plant when he saw a brilliant green line that no known element produced. The discovery was made possible by the newly invented technique of spectroscopy.
In 1862 Claude-Auguste Lamy in France independently isolated thallium as a metal ingot, and the two men argued over priority. Today Crookes is usually credited with the discovery and Lamy with the first isolation of the metal.
Origin of the name
The name comes from the Greek thallos, meaning a green shoot or twig, after the vivid green line in its spectrum.
Main uses
- Nuclear medicine: thallium-201 is taken up by heart muscle much like potassium, so it was widely used in myocardial perfusion scans before and after exercise; technetium-99m agents have largely replaced it.
- Detectors and optics: thallium-doped sodium iodide crystals are classic scintillators for gamma-ray detectors, and thallium compounds are used in high-refractive-index glass and infrared optics.
- Low-temperature thermometers: a mercury-thallium alloy stays liquid down to about -58 °C and was used in thermometers for polar regions.
- Former uses: rat and ant poisons and even a depilatory for hair removal, now banned in most countries after many accidents.
Isotopes
Natural thallium has two stable isotopes, thallium-203 (about 30%) and thallium-205 (about 70%). Medical thallium-201 has a half-life of about 73 hours. Thallium-204, with a half-life of about 3.8 years, has been used in thickness gauges.
In everyday life
Thallium poisoning typically causes abdominal pain and nerve damage, followed weeks later by hair loss. Because it has no taste or smell, it has featured in historical poisoning cases and earned the nickname "the poisoner's poison". The antidote is Prussian blue, a blue pigment given by mouth, which binds thallium in the gut so it can be excreted. Agatha Christie described thallium poisoning so accurately in her novel The Pale Horse that readers are said to have recognised real cases from it. Ordinary people rarely encounter thallium today, but old pesticides and laboratory reagents should be disposed of through specialist services.
Good to know
- Thallium was named "green shoot" after the bright green line in its spectrum.
- A hallmark of thallium poisoning is hair loss that appears weeks after exposure.
- Prussian blue, a painter's pigment, is the antidote for thallium poisoning.
Same group (13)
Data sources · Properties: PubChem Periodic Table (US NIH/NLM public data) · Atomic weights: CIAAW 2024 abridged standard atomic weights · Names: Wikidata (CC0); Korean names follow the Korean Chemical Society. Retrieved 2026-09-23.