They call it the "poisoner's poison." Thallium is a soft, silvery metal with no taste and no smell — yet a dose the size of a small coin can kill a person. That made it a weapon of choice in dozens of criminal cases worldwide. Agatha Christie even featured it in one of her mystery novels.
But thallium is more than just a poison. It was discovered through beauty: a brilliant green spectral line captivated chemist William Crookes in 1861. Today, the radioisotope thallium-201 saves lives — doctors use it to diagnose heart disease. The same element can be both a killer and a lifesaver.
Thallium belongs to the post-transition metals in group 13. It is so soft you can cut it with a knife. In air, it quickly oxidizes and turns dark. Its density is 11.85 g/cm³ — nearly as heavy as lead. Thallium is more toxic than lead and mercury combined: it penetrates skin easily and accumulates in the body.
The main sources of thallium are byproducts of zinc, lead, and copper smelting. About 10 tons are produced globally each year, mostly in China and Kazakhstan.
Thallium and its compounds are extremely toxic — more poisonous than lead or mercury. It absorbs easily through skin, by inhalation, and through ingestion. Symptoms appear hours or days later: nausea, pain, hair loss, nerve and kidney damage. The lethal dose is about 1 gram. The antidote is Prussian blue. Working with thallium requires sealed systems, fume hoods, protective clothing, and strict monitoring.
Thallium is more toxic than lead and mercury. The lethal dose for a human is about 1 gram — that's less than a single grape weighs.
The name 'thallium' comes from Greek thallos — 'green shoot.' William Crookes saw a vivid green spectral line and was captivated by it.
Agatha Christie described thallium poisoning in her novel The Pale Horse (1961). After publication, the book actually helped doctors diagnose a real poisoning case.
The radioisotope thallium-201 is used for heart scans. It reveals areas where heart muscle receives too little blood — helping detect ischemia.
The antidote for thallium poisoning is Prussian blue — a bright blue pigment that binds thallium in the intestines and flushes it from the body.
Thallium is a byproduct of zinc and lead smelting. There are no dedicated 'thallium mines.' Annual global production is just 10 tons for the entire planet.
| Isotope | Mass (u) | Abundance | Half-life | Decay |
|---|---|---|---|---|
203Tl | 202.972344 | 29.52% | stable | — |
205Tl | 204.974427 | 70.48% | stable | — |
Spectroscopic analysis of selenium residue