Thallium (Tl)

Thallium is a chemical element with the atomic number 81 in the periodic table. It’s a moderately abundant metal, found in trace amounts in Earth’s crust. In fact, thallium is 10 times more abundant than silver. Being a member of the aluminum family of periodic table elements, this post-transition metal has three valence electrons and is extremely toxic. Namely, this soft and easily melted substance has an especially strong effect on the central nervous system in humans, which can lead to paralysis and severe damage to the brain. 

Thallium-bearing minerals are rare in nature. The Alchar mine in the Republic of North Macedonia is the only location in the world where the thorium-rich mineral lorandite occurs in its purest form. Scientists are especially intrigued by it as a geochemical detector of solar neutrinos. If the power of the Sun could be thoroughly understood with the help of lorandite, the scientists would be able to produce many other “mini-Suns” that would produce environmentally clean power for our planet.

Chemical and Physical Properties of Thallium

Atomic number81
Period6 (p-block)
ColorA bluish-white hued metal
Physical stateSolid at room temperature
Half-lifeFrom 5.2(+30−14) milliseconds to 3.78 years
Melting point304°C, 579°F, 577 K
Boiling point1473°C, 2683°F, 1746 K
Van der Waals radius0.182 nm
Ionic radius1.59 (+1) Å
Atomic weight204.3833
Most characteristic isotope205Tl
Electronic shell[Xe] 4f14 5d10 6s2 6p1
The energy of the first ionization589.1 kJ.mol-1
The energy of the second ionization1970.5 kJ.mol-1
The energy of the third ionization2877.4 kJ.mol-1
Crystal StructureHexagonal
Covalent Radius1.48 Å
Atomic Radius2.08 Å
Atomic Volume17.2 cm³/mol
Name OriginGreek: thallos (green twig)
Discovery date1861
DiscovererSir William Crookes
Location of DiscoveryEngland
Oxidation States3,(1)
UsesRat and ant poisons, detecting infrared radiation
DescriptionVery soft, malleable, ductile, blue-white shiny metal

With the periodic table symbol Tl, atomic number 81, atomic mass of X g.mol-1, and electron configuration [Xe] 4f145d106s26p1, thallium is a soft, tasteless, odorless, malleable, and lustrous metal. Any sharp or hard object could make a dent in it. 

Thallium reaches its boiling point at 1473°C, 2683°F, 1746 K, while the melting point is achieved at 304°C, 579°F, 577 K. This member of the boron family of elements in the periodic table has an electronegativity of 1.8 according to Pauling, whereas the atomic radius according to van der Waals is 0.182 nm. 

Element 81 has a hexagonal close-packed structure. When thallium reacts with nitric acid and sulfuric acid, it forms nitrate and sulfate salts. It dissolves slowly in both hydrochloric acid and diluted sulfuric acid, while this chemical reaction is more rapid in nitric acid.

In its pure, elemental form, thallium metal appears gray in color, resembling the element tin in its physical appearance. However, when this post-transition metal is exposed to high temperature, it becomes discolored.

How Was Thallium Discovered?

In 1861, element 81 was independently discovered by the French chemist Claude-Auguste Lamy (1820-1878) and the British chemist and physicist Sir William Crookes (1832-1919).

When, in 1850, Sir William Crookes attempted to extract selenium from a sample obtained from a sulfuric acid factory in Tilkerode, he observed a tellurium concentration in the residue of the chemical reaction. A decade later, Sir Crookes conducted an experiment for which he needed some tellurium. The British chemist recalled that the selenium isolation process resulted in some tellurium quantities as a residue, so he tried to recreate the chemical process. Strangely enough for him, this time he wasn’t able to produce tellurium. 

Eager to get to the root of this chemical enigma, Sir Crookes applied the method of spectroscopic analysis in an effort to understand where the experiment went wrong. By the means of flame spectroscopy, he observed a bright green line in the spectrum that hadn’t been seen before in any other known chemical element. Confident in his scientific results, Sir William Crookes published his findings in a journal he edited and published, called Chemical News.

Despite being unable to discover all of thallium’s physical and chemical properties, Sir Crookes succeeded in producing thallium salts that he showcased at the International Exhibition held in South Kensington, London, in 1862.

As chance would want it, the French chemist Claude-Auguste Lamy was also attending the London exhibition. Since he succeeded in isolating thallium in larger quantities the same year, Lamy brought an ingot of pure thallium metal to the exhibition. By this, this French chemist became the first scientist to isolate the pure metal form of element 81. 

How Did Thallium Get Its Name?

The name thallium originates from the Greek word ‘thallos‘, meaning a ‘green shoot or green twig’. Sir William Crook chose this name for his new element, due to its characteristic green spectral line that serves as an identification mark of element 81.

Where Can You Find Thallium?

This post-transition metal is not a naturally occurring chemical. It can be found in granites, soils, and clays. Thallium also appears in metal-sulfide ores, pyrite, zinc, lead, copper, as well as in minerals that contain the chemical elements silver, lead, copper, arsenic, antimony, and other heavy-metal sulfide ores. 

While the mineral lorandite contains thallium in the purest form, there are also other thallium-bearing minerals, such as crookesite, sabatierite, routhierite, christite, bukovits, hutchinsonite, and gabrielite. Thallium obtained from these minerals requires a long process of isolation and purification than the thallium isolated from lorandite ore. 

The mineral lorandite can be found only in a mine located in the Republic of North Macedonia, while the gabrielite mineral is found in Switzerland. The mineral deposits in Nevada, United States, and China contain some quantities of the christite mineral. In addition, bukovite is the most dispersed thallium-bearing mineral and occurs in deposits in Argentina, Sweden, France, and the Czech Republic.

For commercial purposes, thallium can be synthesized by smelting zinc and lead. It can also be obtained as a by-product of the sulphuric acid production process or extracted from dust from the smelter flues.

The Alchar Mine

The largest and richest concentration of thallium minerals in the world is found in the Alchar mine (Macedonian: Алшар) on Kozuf, a mountain with an untouched natural abundance of rare minerals as well as indigenous botanical and animal species. 

Dating back to the 14–15 centuries, this thallium mine is a low-temperature hydrothermal gold–arsenic–antimony–thallium deposit, located in the heart of the southern Balkan country. Existing for more than five millennia, it was the only mine in the world where thallium was actively mined until World War II. 

The ancient mine is nowadays declared a protected site and it is accessible only to scientists working on the geochemical LORAX project that holds special approval from the North Macedonian government officials as well as from the Sts. Cyril and Methodius University in Skopje. 

List of Thallium Minerals

In 1894, the most common thallium-bearing mineral lorandite (lorándite) was first discovered by the Hungarian mineralogist József Sándor Krenner in the Alchar mine, Republic of North Macedonia. By its composition, this mineral is a thallium arsenic sulfosalt (TlAsS2). It’s assumed that this unique mineral was formed only at this place on our planet after an explosion on the Sun that has occurred around a billion years ago. Lorandite is considered to be one of the rarest minerals in the world.

Apart from the lorandite as the main mineral in which element 81 occurs, thallium can be also traced in the following minerals: 

  • Avicennite
  • Bukovite
  • Christite
  • Chrysothallite
  • Crookesite
  • Enneasartorite
  • Gabrielite
  • Hendekasartorite
  • Heptasartorite
  • Honeaite
  • Hutchinsonite
  • Routhierite
  • Sabatierite

While the Alchar mine was the only place in the world where thallium could be obtained until WWII, today small quantities of element 81 can also be traced in Tajikistan, Russia, China, Iran, Switzerland, and United States (New Rambler Cu–Ni mine in Wyoming, Jerritt Canyon mines in Nevada, and Mercur gold mine in Utah). In these deposits, thallium typically occurs alongside gold, silver, zinc, selenium, lead, and copper ores.

Thallium in Everyday Life

Everyday uses of thallium in the past and in the present differ immensely. Namely, thallium was mostly used as an insecticide and rodenticide or in skin treatments in the past. Nowadays the thallium compounds have limited practical use due to their extreme toxicity, while thallium metal has no practical application yet.

  • The most popular modern use of thallium is in the production of electronic devices, fiber optics, camera lenses, switches, closures, mercury lamps, as well as in high-temperature superconductors.
  • Since the conductivity of thallium sulfide is transformed when exposed to infrared light, thallium is an important compound used in the manufacturing of photocells.
  • Element 81 is often employed as a catalyst for organic reactions.
  • It’s also used In the production of specialized extreme-temperature glasses and low-temperature thermometers;
  • Small amounts of thallium can be also found in cigarette smoke, fireworks, pigments, and jewelry;
  • Thallium gives a brilliant green flame to the Bunsen burner.
  • In the past, thallium was popularly used as a rat poison and an ant killer substance; 
  • Radioactive isotopes of thallium have special use in radiography for early detection of cardiovascular problems;
  • Before its toxicity was discovered, thallium was widely used in the treatment of some medical diseases, such as malaria, tuberculosis, typhus, ringworm of the scalp, and venereal diseases.

How Dangerous Is Thallium?

According to the CDC and the U.S. Agency for Toxic Substances and Disease Registry, thallium and thallium compounds are classified as extremely poisonous substances. The toxicological profile of this chemical element displays a wide array of adverse health effects triggered by exposure to any level of thallium. 

If ingested, even several milligrams of thallium can be lethal. According to the National Institute for Occupational Safety and Health (NIOSH) recommendations, even 15 mg/m³ of thallium should be considered immediately dangerous to life and health.

Thallium Poisoning

Exposure to thallium via inhalation, skin contact, or ingestion may lead to thallium poisoning. Thallium levels in urine, blood, and hair are the common indications of exposure to thallium. The toxicity of this chemical element mainly affects the central nervous system. This further leads to an occurrence of the characteristic physical and neurological symptoms of thallium poisoning, which include:

  • Hair loss or Alopecia (often described in Agatha Christie’s mysteries);
  • Severe headache;
  • Mees lines (horizontal white depositions in the nails);
  • Vomiting;
  • Hallucinations;
  • Loss of consciousness;
  • Lung obstruction;
  • Gastrointestinal problems.

Case reports on thallium poisoning show that the affected individuals who have been exposed to thallium may endure complications of these symptoms, which include: 

  • Paralysis;
  • Nerve damage;
  • Involuntary trembling;
  • Hallucinations;
  • Lethargy;
  • Delirium;
  • Anemia;
  • Enlarged spleen;
  • Convulsions;
  • Grierson-Gopalan syndrome;
  • Coma;
  • Death. 

Physical contact with rodenticides (rat poison) is the most frequent reason behind acute thallium poisoning and exposure. However, most of the aforementioned symptoms could also be associated with arsenic poisoning, which also occurs after inhalation of insecticides. In addition, it should be noted that thallium poisoning may also occur in smokers, due to the presence of thallium in the tobacco

Prussian Blue is a potassium-rich oral cation that has been approved by FDA as an antidote for cesium and thallium intoxication. Hemodialysis and hemoperfusion are also used to remove thallium from the blood. Regardless of the severity of symptoms, all patients suffering from thallium poisoning should have close follow-up care.

Environmental Effects of Thallium

Since this chemical element mostly occurs in the sulfide ores of other metals, thallium concentration in the environment is generally on the low side. However, the areas surrounding the industries working with this toxic substance (such as smelters, coal-burning power plants, cement plants, electronics industry) have a significantly higher concentration of element 81. 

Isotopes of Thallium

There are 41 isotopes of thallium with atomic masses ranging from 176Tl to 216Tl. Natural thallium consists of two stable isotopes: thallium-203 (29.5 percent) and thallium-205 (70.5 percent). Isotopes thallium-203 and thallium-205 are the most stable form of element 81. 

Apart from the naturally occurring isotopes, some forms of thallium can be synthesized. For instance, thallium-202 from this chemical element can be produced in a cyclotron, while the thallium-204 isotope can be obtained by a neutron activation process of stable thallium isotope in a nuclear reactor. 

The radioactive forms of thallium occur as decay products of three natural radioactive disintegration series: thallium-206 and thallium-210 (uranium series), thallium-208 (thorium series), and thallium-207 (actinium series).


[n 1]



ZNIsotopic mass (Da)[6]

[n 2][n 3]


[n 4]



[n 5]



[n 6]

Spin and


[n 7][n 4]

Natural abundance (mole fraction)
Excitation energy[n 4]Normal proportionRange of variation
176Tl 8195176.00059(21)#5.2(+30−14) ms  (3−, 4−, 5−)  
177Tl 8196176.996427(27)18(5) msp176Hg(1/2+)  
α (rare)173Au
178Tl 8197177.99490(12)#255(10) msα174Au   
p (rare)177Hg
179Tl 8198178.99109(5)270(30) msα175Au(1/2+)  
p (rare)178Hg
180Tl 8199179.98991(13)#1.5(2) sα (75%)176Au   
β+ (25%)180Hg
EC, fission (10−4%)100Ru, 80Kr[7]
181Tl 81100180.986257(10)3.2(3) sα177Au1/2+#  
182Tl 81101181.98567(8)2.0(3) sβ+ (96%)182Hg2−#  
α (4%)178Au
183Tl 81102182.982193(10)6.9(7) sβ+ (98%)183Hg1/2+#  
α (2%)179Au
184Tl 81103183.98187(5)9.7(6) sβ+184Hg2−#  
185Tl 81104184.97879(6)19.5(5) sα181Au1/2+#  
186Tl 81105185.97833(20)40# sβ+186Hg(2−)  
α (.006%)182Au
187Tl 81106186.975906(9)~51 sβ+187Hg(1/2+)  
α (rare)183Au
188Tl 81107187.97601(4)71(2) sβ+188Hg(2−)  
189Tl 81108188.973588(12)2.3(2) minβ+189Hg(1/2+)  
190Tl 81109189.97388(5)2.6(3) minβ+190Hg2(−)  
191Tl 81110190.971786(8)20# minβ+191Hg(1/2+)  
192Tl 81111191.97223(3)9.6(4) minβ+192Hg(2−)  
193Tl 81112192.97067(12)21.6(8) minβ+193Hg1/2(+#)  
194Tl 81113193.97120(15)33.0(5) minβ+194Hg2−  
α (10−7%)190Au
195Tl 81114194.969774(15)1.16(5) hβ+195Hg1/2+  
196Tl 81115195.970481(13)1.84(3) hβ+196Hg2−  
197Tl 81116196.969575(18)2.84(4) hβ+197Hg1/2+  
198Tl 81117197.97048(9)5.3(5) hβ+198Hg2−  
199Tl 81118198.96988(3)7.42(8) hβ+199Hg1/2+  
200Tl 81119199.970963(6)26.1(1) hβ+200Hg2−  
201Tl[n 8] 81120200.970819(16)72.912(17) hEC201Hg1/2+  
202Tl 81121201.972106(16)12.23(2) dβ+202Hg2−  
203Tl 81122202.9723442(14)Observationally Stable[n 9]1/2+0.2952(1)0.29494–0.29528
204Tl 81123203.9738635(13)3.78(2) yβ (97.1%)204Pb2−  
EC (2.9%)204Hg
205Tl[n 10] 81124204.9744275(14)Observationally Stable[n 11]1/2+0.7048(1)0.70472–0.70506
206TlRadium E81125205.9761103(15)4.200(17) minβ206Pb0−Trace[n 12] 
207TlActinium C81126206.977419(6)4.77(2) minβ207Pb1/2+Trace[n 13] 
208TlThorium C”81127207.9820187(21)3.053(4) minβ208Pb5+Trace[n 14] 
209Tl 81128208.985359(8)2.161(7) minβ209Pb1/2+Trace[n 15] 
210TlRadium C″81129209.990074(12)1.30(3) minβ (99.991%)210Pb(5+)#Trace[n 12] 
β, n (.009%)209Pb
211Tl 81130210.993480(50)80(16) sβ (97.8%)211Pb1/2+  
β, n (2.2%)210Pb
212Tl 81131211.998340(220)#31(8) sβ (98.2%)212Pb(5+)  
β, n (1.8%)211Pb
213Tl 81132213.001915(29)24(4) sβ (92.4%)213Pb1/2+  
β, n (7.6%)212Pb
214Tl 81133214.006940(210)#11(2) sβ (66%)214Pb5+#  
β, n (34%)213Pb
215Tl 81134215.010640(320)#10(4) sβ (95.4%)215Pb1/2+#  
β, n (4.6%)214Pb
216Tl 81135216.015800(320)#6(3) sβ216Pb5+#  
β, n (<11.5%)215Pb

Source: Wikipedia

List of Thallium Compounds

In a compound, thallium typically adopts the oxidation states +1, +2, and +3. There are two crystalline forms of this chemical element: a close-packed hexagonal structure (below 230 °C / 450 °F) and a body-centered cubic structure (above 230 °C / 450 °F).

All soluble thallium compounds (especially thallium salts) are regarded as some of the most toxic compounds known today and are suspected as carcinogenic. 

The most commonly prepared thallium compounds are listed below:

  • Clerici solution
  • Thallium cyclopentadienide
  • Thallium azide
  • Thallium triiodide
  • Thallium(I) bromide
  • Thallium(I) carbonate
  • Thallium(I) chloride
  • Thallium(I) fluoride
  • Thallium(I) hydroxide
  • Thallium(I) iodide
  • Thallium(I) oxide
  • Thallium(I) sulfate
  • Thallium(I) sulfide
  • Thallium(I) telluride
  • Thallous acetate
  • Thallous malonate
  • Thallane
  • Thallium(III) hydroxide
  • Thallium(III) nitrate
  • Thallium(III) oxide
  • Thallium barium calcium copper oxide

5 Interesting Facts and Explanations

  1. The mineral lorandite, from which thallium is primarily obtained, is named after the distinguished Hungarian physicist Baron Loránd Eötvös de Vásárosnamény (1848-1919). Lorandite is one of the rarest minerals in the world.
  2. Agatha Mary Clarissa Christie is an English novelist, world-famous for her detective novels. In her writing, Dame Christie depicts crime cases that often revolve around thallium poisoning. Namely, thallium sulfate is depicted as the main cause of the mysterious deaths that the fictional detective Hercule Poirot is to solve.
  3. LOREX is an acronym that refers to the Lorandite Experiment. It’s a scientific project aimed at the detection of the solar neutrino flux and a more precise calculation of the mean luminosity of the Sun over the last 4.3 million years by using lorandite obtained from the Alchar mine.
  4. Solar neutrinos are neutral subatomic particles of the Sun. By the force of its chemical and physical properties, the lorandite mineral is able to detect the neutron flux coming from the Sun after a nuclear fusion. Their weight is almost zero, and they travel with the speed near the one of the light. Solar neutrinos reach planet Earth before sunlight does. Because they rarely interact with matter, the neutral subatomic particles of the Sun are first to bolt out of the Sun’s dense core, while the light particles (photons) fluctuate around it a bit longer before they are released as sun rays. We can easily learn about the core of our Sun and its power from these solar neutrinos.
  5. Scientists from all over the world employ the Alchar lorandite as a geochemical detector of solar neutrinos for its exceptionally high purity of composition. By conducting the scientific trials with this rare mineral, they are making extensive efforts for a better understanding of the standard solar system, but also of the past and the future of our Galaxy.