Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where thallium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Size, to scale
Radius 170 pm — that is 0.17 nm, so about 2941 million of them side by side would span a millimetre.
Thermal range
Thallium is liquid over a 1169 K window, from 577 K to 1746 K.
The story
Colourless, tasteless and lethal — thallium was the poisoner's element until detection caught up.
The discovery of thallium reads like a detective story that began in March 1861 at the Royal College of Chemistry in London. Sir William Crookes, a brilliant chemist and physicist, was investigating the residues from sulfuric acid production when he made an observation that would change chemistry for very long periods.
Using the newly invented spectroscope - a device that splits light into its component colors - Crookes was examining samples from the Tilkerode mine in the Harz Mountains of Germany. As he heated the sample in the spectroscope flame, he noticed something extraordinary: a brilliant green line at 535 nanometers that had never been seen before.
"The beauty of this green line," Crookes later wrote, "was so intense that it immediately attracted my attention." This single green line was the fingerprint of a completely unknown element.
Crookes named his new element "thallium" from the Greek word "thallos" meaning "green shoot" or "green twig," in honor of the distinctive green spectral line that revealed its presence. This name perfectly captured the element's most characteristic property - its brilliant green flame color.
Interestingly, Crookes initially thought he had discovered a new metal similar to lead, having no idea he had found one of the most toxic elements on Earth.
Just months after Crookes' discovery, Claude-Auguste Lamy in France independently discovered thallium using similar spectroscopic techniques. This led to a brief scientific dispute over priority, but both men were ultimately credited with the discovery.
Lamy actually managed to isolate metallic thallium first in 1862, producing enough pure metal to study its properties. He found it to be surprisingly soft - softer than lead - and noted its high density. Neither scientist initially realized they were handling one of nature's most dangerous elements.
The early researchers faced significant challenges studying thallium. The element proved difficult to isolate in pure form, and its compounds showed puzzling chemical behavior - sometimes acting like alkali metals, other times like heavy metals.
Dmitri Mendeleev initially had trouble placing thallium in his periodic table, as its properties didn't fit neatly into existing patterns. The element seemed to bridge the gap between alkali metals and heavy metals, a property we now understand is due to relativistic effects on its electrons.
The true nature of thallium's toxicity wasn't discovered until the 1890s, when several researchers experienced mysterious illnesses after working with the element. Hair loss, nerve damage, and gastrointestinal problems plagued early thallium researchers.
The most tragic case involved Dr. Marie Curie's assistant, who suffered severe thallium poisoning in 1904 while attempting to purify thallium compounds. This incident led to the first safety protocols for handling thallium, though it would be decades before its extreme toxicity was fully understood.
By the 1920s, thallium's reputation had completely transformed from "Crookes' beautiful green element" to "the poisoner's poison," earning it nicknames like "inheritance powder" due to its use in criminal poisonings.
Today, Crookes is remembered not just for discovering thallium, but for pioneering spectroscopic analysis in chemistry. His work with thallium demonstrated the power of spectroscopy to reveal hidden elements, a technique that would go on to discover helium, cesium, rubidium, and many others.
The discovery of thallium also contributed to our understanding of atomic structure and relativistic effects in heavy atoms, making it an important element in theoretical chemistry despite its limited practical applications.
Applications
Thallium's unique electronic properties make it valuable in specialized semiconductors. Thallium bromide-iodide (TlBr-TlI) crystals are used in infrared detectors and gamma-ray spectrometers. These detectors operate at room temperature, unlike many alternatives requiring cooling, making them ideal for portable radiation detection equipment used by nuclear security agencies.
Thallium-doped sodium iodide crystals serve as scintillators in medical imaging equipment, converting gamma rays into visible light with exceptional efficiency. The addition of thallium significantly improves light output compared to pure sodium iodide.
Thallium bromoiodide (KRS-5) windows are essential components in infrared spectroscopy. These crystals transmit infrared radiation from 0.5 to 40 micrometers, making them central for FTIR spectroscopy, environmental monitoring, and chemical analysis. Despite their toxicity, no suitable replacement exists for many applications.
High-refractive-index thallium glass is used in specialized optical components where extreme light-bending properties are required, though safety concerns have limited widespread adoption.
In superconductivity research, thallium-barium-calcium-copper oxide compounds hold records for high-temperature superconductivity. Tl-2223 (Tl₂Ba₂Ca₂Cu₃O₁₀) maintains superconductivity at temperatures up to 125 K (-148°C), making it valuable for research into room-temperature superconductors.
Thallium-201 radioisotope is produced in cyclotrons for nuclear medicine, particularly cardiac imaging. Its 72-hour half-life and gamma emission properties make it ideal for stress tests and heart muscle visualization.
Historically, thallium compounds were used as rodenticides and insecticides due to their extreme toxicity. Thallium sulfate was marketed as "Zellio" and other brand names until widespread poisoning incidents led to bans in most countries by the 1970s.
Thallium was once added to optical glass to increase refractive index, but safety concerns ended this practice. Some vintage camera lenses still contain thallium glass, requiring careful handling during repairs.
All thallium applications require extreme safety precautions. Even microscopic amounts can cause severe poisoning. Modern uses are limited to specialized applications where no alternatives exist and strict containment is possible.
Thallium-201 heart scans remain a gold standard for detecting coronary artery disease. Patients receive a tiny injection of Tl-201, which accumulates in healthy heart muscle. Areas with poor blood flow appear as "cold spots" on gamma camera images, helping doctors diagnose blockages before heart attacks occur.
Radiation detectors containing thallium-activated crystals are used in nuclear security, environmental monitoring, and space research. These detectors can identify specific radioactive materials, making them crucial for preventing nuclear terrorism and monitoring reactor safety.
Infrared windows made from thallium compounds are essential for gas analysis instruments used in pollution monitoring, industrial process control, and atmospheric research.
From 1920-1970, thallium sulfate was widely used as "rat poison" under names like Zellio, Ratox, and Thall-rat. Its effectiveness came from being colorless, odorless, and tasteless - the same properties that made it dangerous to humans. Accidental poisonings and criminal use led to worldwide bans.
Incredibly, thallium was once used to treat ringworm and other fungal infections in the early 1900s. The treatment caused hair loss (still seen in some chemotherapy research), but often resulted in severe poisoning. This practice was abandoned by the 1930s as safer alternatives became available.
Thallium is now heavily regulated worldwide. In the US, it's controlled by the EPA as an extremely hazardous substance. Most countries prohibit its sale to private individuals. Even research institutions require special licenses and disposal protocols.
The few remaining uses are limited to:
⚠️ Public Safety Warning: Never attempt to purchase or handle thallium compounds. Even touching contaminated surfaces can cause severe poisoning. If you suspect thallium exposure, seek immediate medical attention.
Where it comes from
0.85 mg/kg of Earth's crust · more abundant than 50% of elements
Thallium is one of the rarest stable elements in Earth's crust, with an average abundance of only 0.7 parts per million. It never occurs as a pure metal in nature due to its high reactivity and forms no major ore minerals of its own.
Instead, thallium is found as a trace constituent in sulfide minerals, particularly:
Kazakhstan produces about 60% of the world's thallium supply, primarily as a byproduct of zinc smelting at the Balkhash complex. The thallium is recovered from flue dusts and residues during zinc refining.
China is the second-largest producer, extracting thallium from lead-zinc ores in Yunnan and Hunan provinces. Chinese production has grown significantly due to increased demand for electronics applications.
Smaller amounts are produced in:
Thallium concentrates in coal deposits, with some coals containing up to 15 ppm. Coal burning releases thallium into the atmosphere, making it a global environmental pollutant. This is why thallium levels in the environment have increased since the Industrial Revolution.
Volcanic activity naturally releases thallium compounds into the atmosphere. Some volcanic regions show elevated thallium levels in soil and groundwater, though typically still at trace levels.
Marine environments contain extremely low thallium concentrations (about 10-15 nanograms per liter), but some deep-sea organisms can bioconcentrate it, leading to concerns about seafood safety in polluted areas.
Most commercial thallium comes from flue dusts collected during the smelting of zinc, lead, and copper ores. These dusts are processed through complex hydrometallurgical procedures to extract and purify thallium compounds.
The process involves:
Due to extreme toxicity, all processing occurs in sealed systems with extensive safety protocols and waste treatment facilities.
To put thallium's rarity in perspective: it's about three times rarer than gold in Earth's crust. Annual global production is only about 10-15 tons, compared to thousands of tons for most industrial metals. This extreme rarity, combined with its toxicity, keeps prices high and applications limited.
Handling
Thallium is one of the most toxic elements known to science. Even microscopic exposure can cause severe poisoning and death.
Lethal dose: As little as 0.5-1.0 grams can kill an adult human. For comparison, this is roughly the mass of a paperclip. Children are even more susceptible, with fatal doses as low as 8-10 mg/kg body weight.
Absorption routes: Thallium compounds are rapidly absorbed through skin, lungs, and digestive system. Even brief contact with contaminated surfaces can cause poisoning.
Symptoms appear slowly: Unlike many poisons, thallium poisoning develops over days to weeks, making early detection difficult. Initial symptoms mimic flu or food poisoning.
Early symptoms (1-3 days): Nausea, vomiting, diarrhea, abdominal pain, fatigue. Often misdiagnosed as gastroenteritis.
Progressive symptoms (1-2 weeks): Complete hair loss (alopecia), peripheral neuropathy causing severe pain and numbness in hands and feet, confusion, memory problems.
Advanced poisoning: Respiratory failure, cardiac arrhythmias, kidney failure, coma, death. Survivors often suffer permanent neurological damage.
Diagnostic marker: Total hair loss is pathognomonic (characteristic) of thallium poisoning and appears 2-3 weeks after exposure.
Suspected exposure: Seek immediate medical attention. Do not wait for symptoms. Contact poison control: 1-800-222-1222 (US).
Decontamination: Remove contaminated clothing carefully. Wash skin with soap and water for at least 15 minutes. Do not induce vomiting if ingested.
Medical treatment: Prussian blue (ferric ferrocyanide) is the specific antidote for thallium poisoning. Early administration is critical for survival.
Do not use activated charcoal - it is ineffective against thallium and may delay proper treatment.
Personal Protection: Full body protection including supplied-air respirator, chemical-resistant gloves, and protective clothing. Never work alone.
Containment: All work must be conducted in negative pressure containment with HEPA filtration. No eating, drinking, or smoking in work areas.
Waste disposal: All potentially contaminated materials must be treated as hazardous waste. Special disposal protocols required by environmental agencies.
Monitoring: Regular medical surveillance including urine testing for workers with potential exposure.
Thallium is heavily regulated worldwide. In the US, it's listed as an extremely hazardous substance under RCRA. Most countries restrict sales to licensed facilities only. Criminal penalties apply for unauthorized possession or use.
Quick answers
Thallium (symbol Tl) is element 81 on the periodic table, a post-transition metal in period 6, group 13. Colourless, tasteless and lethal — thallium was the poisoner's element until detection caught up. At room temperature it is a solid.
[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹, giving 6 occupied shells holding 2, 8, 18, 32, 18, 3 electrons respectively. Its outer shell holds 3 electrons, which is what sets its bonding behaviour.Thallium melts at 577 K (303.9 °C) and boils at 1746 K (1472.9 °C).
The standard atomic weight of Thallium is 204.38 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
Thallium has a density of 11.85 g/cm³. Water is 1.0 g/cm³, so a block of thallium is about 11.9× heavier.
Thallium has a Pauling electronegativity of 1.62. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.
Thallium was discovered in 1861 by William Crookes. It is named after greek thallos, "green shoot" — its spectral line.
Thallium makes up about 0.85 mg/kg of the Earth's crust — genuinely rare, which is why it is expensive.