Thallium · element 81
A soft grey metal named after a green twig, and so poisonous it was nicknamed the poisoner's poison
Key facts
Thallium looks unremarkable, a soft, bluish grey metal that dulls quickly in air and can be cut with a knife. Its name comes from the Greek thallos, a green shoot, after the brilliant green line it produces when heated in a flame. Behind that gentle name lies one of the most toxic metals known.
Thallium compounds are tasteless and odourless, their effects are delayed by days, and the symptoms mimic other illnesses. That combination made thallium a favourite of real and fictional poisoners for most of the twentieth century, and it is now banned as a pesticide in most of the world.
Where it is found
Thallium is rare, less than a part per million of the crust, and seldom gathers into minerals of its own; a few, such as crookesite and lorandite, are collectors' curiosities. Instead it is scattered in trace amounts through sulfide ores of zinc, copper and lead, and through coal.
Almost all commercial thallium comes from the dusts collected in the flues of smelters that roast these ores. World production is small, around ten tonnes a year, mainly from China, Kazakhstan and Russia. Far more thallium is released unintentionally by coal burning and cement making than is ever deliberately produced.
How it was discovered
Thallium was one of the first elements found with a spectroscope, an instrument barely a year old at the time. In 1861 William Crookes, working in London, was examining residues from a sulfuric acid factory in search of tellurium when he saw in the flame spectrum a bright green line that matched nothing known, and announced a new element.
Across the Channel the French chemist Claude-Auguste Lamy found the same green line independently in 1862 and went further, isolating the metal and describing its properties. The two men argued over priority for years. Crookes went on to invent the Crookes tube, the evacuated glass tube that led to the discovery of X-rays and the electron.
What we use it for
Thallium's first big market was as a rat and ant poison, and in the 1930s a thallium cream was even sold to remove unwanted hair, with disastrous results. Household use was banned in the United States in the 1970s and in most other countries around the same time.
Modern uses are small and specialised. Sodium iodide crystals doped with a trace of thallium are standard detectors for gamma rays, flashing with light when radiation passes through. The radioactive isotope thallium-201 is injected into patients to image blood flow to the heart. Crystals of thallium bromide and iodide transmit infrared light for specialist optics, a mercury-thallium alloy stays liquid to about minus 60 °C and once filled low temperature thermometers, some high temperature superconductors contain thallium, and dense thallium solutions help gemmologists separate minerals by density.
Reading the numbers
Thallium melts at about 304 °C, not far above the hottest setting of a domestic oven, and boils near 1470 °C. Its density of 11.85 grams per cubic centimetre is a little higher than lead, nearly twelve times that of water. Soft, heavy and low melting, it is typical of the metals in the lower right of the table.
Its electron configuration, [Xe] 4f14 5d10 6s2 6p1, has three outer electrons, placing it in group 13 beneath boron, aluminium, gallium and indium. Those lighter relatives use all three electrons and form +3 ions. Thallium usually gives up only the single 6p electron and forms Tl+, because the 6s pair is held unusually tightly in such a heavy atom, an effect chemists call the inert pair effect. The first ionisation energy of 589 kJ/mol is low and the electronegativity of 1.62 is modest, marking a reactive metal. The Tl+ ion happens to be almost the same size as a potassium ion, and that coincidence is the root of its toxicity.
Safety and the environment
The body cannot tell thallium from potassium, so it is absorbed through the gut, lungs and even unbroken skin, and carried into every cell where it wrecks the machinery that potassium normally runs. Less than a gram can kill an adult. Early symptoms are stomach pain and burning nerve pain in the feet; the tell-tale sign, hair falling out, appears two to three weeks later. The antidote is the pigment Prussian blue, taken by mouth, which binds thallium in the gut.
In the environment thallium leaks from coal ash, smelter waste and some cement works, and is taken up readily by certain plants, especially the cabbage family. Levels in food are monitored near old mines and smelters.
Did you know?
- Thallium poisoning was famously used in Agatha Christie's novel The Pale Horse, and in the 1970s a real case in London was diagnosed after a nurse recognised the symptoms from the book.
- The antidote to thallium poisoning is Prussian blue, the same pigment artists have used since the eighteenth century.
- Thallium's name means green twig, after the bright green line it produces in a flame spectrum.
- Thallium was one of the first elements discovered using the spectroscope, only about a year after the technique was invented.
- The isotope thallium-201 is used in hospitals to image blood flow to the heart muscle.
- A mercury-thallium alloy stays liquid down to about minus 60 °C, twenty degrees colder than pure mercury.
More in the post-transition metals
Column-mates come first, since elements in one column have outer electrons arranged in a similar way and tend to behave alike; the rest are the nearest neighbours in the family.
See all 12 post-transition metals on one page, or browse all 118 elements.
Frequently asked questions
Why is thallium so poisonous?
Thallium ions are almost identical in size and charge to potassium ions, so the body absorbs them eagerly and delivers them into every cell. Once inside they disrupt the enzymes and nerve signals that depend on potassium. The effects are delayed, which makes poisoning hard to spot.
Is thallium still used in rat poison?
Not legally in most countries. Household thallium poisons were banned in the United States in the 1970s and have been prohibited across Europe and much of the world. Illegal and old stocks occasionally still cause poisonings.
What is thallium used for today?
Its uses are small and technical: radiation detector crystals, infrared optics, some high temperature superconductors, and the medical isotope thallium-201 for heart scans. Total world demand is only a few tonnes a year.
How is thallium poisoning treated?
Doctors give Prussian blue by mouth, which traps thallium in the digestive system so it leaves the body in the stool. Supportive care and sometimes blood filtering are also used. Recovery can take months, and nerve damage may be permanent.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Thallium reference article. Spotted an error? Tell us.