Skip to Main Content

About Thallium

Thallium Bohr

Thallium is the heaviest member of Group 13 on the periodic table, a family that consists of the post-transition “poor metals.” The element is a metal that has a low melting point and is extremely malleable, soft enough to be cut with a knife. Pure elemental thallium is initially lustrous and silvery-gray; however, immediately upon exposure to air the metal begins to form a dull bluish-gray film of thallium(I) oxide on its surface. Prolonged exposure converts the film to a dark brown crust of thallium(III) oxide. When submersed in water, the oxide film disappears and the metal regains its original sheen. Prolongued air and water contact converts the metal to thallium hydroxide. Thallium metal is highly reactive with oxygen and acids and dissolves in nitric acid. French chemist Jean-Baptiste Dumas once called it the "ornithorhynchus paradoxus of metals” (the platypus of metals) because of its similarity to several widely varying elements: lead, silver, potassium, mercury, among others.

There is some debate as to whom the credit for discovering thallium should go. English chemist Sir William Crookes assumed he had obtained tellurium as a byproduct after isolating selenium from residues of sulfuric acid production. When he repeated his experiment years later in 1861, this time focusing on isolating tellurium, he was surprised by the unfamiliar brilliant green lines that appeared in the emission spectrum after burning the sample. This method, known as flame spectroscopy, had been invented several years earlier by chemists Robert Bunsen and Gustav Kirchhoff and used to discover several elements including cesium and rubidium, which they named based on Latin descriptions of the colors of their spectral lines. Fittingly, Crookes named his new element thallium after thallos, Greek for “green twig” or “shoot”. Around the same time, if not earlier, French chemist Claude-Auguste Lamy had identified the element independently from Crookes and isolated it one year later; Lamy, in fact, was more successful at producing the metal than Crookes, exhibiting a pure ingot of thallium at the 1682 London International Exhibition compared to the small amount of powder that Crookes had struggled to produce, and was awarded a medal. Despite this, the discovery of thallium is officially attributed to Crookes thanks to his vocal protest of Lemy’s award.

Thallium is quite scarce in the earth’s crust, at about 0.7 mg/kg, roughly as common as iodine or tungsten. Thallium is not found free in nature, but rather is present in small amounts in ores of iron, copper, heavy-metal sulfides, and in polymetallic rock deposits on the ocean floor. Minerals that contain thallium include crooksite, lorandite, hutchinsonite, pyrites, and orpiment; carlinite (thallium sulfide) is one of the few primarily thallium-based minerals present on earth. Naturally-occurring thallium is composed of two stable isotopes, thallium-203 and -205. 23 radioactive isotopes have also been been synthesized. Thallium’s two oxidation states of +1 and +3 (thallous and thallic, respectively) form compounds with very divergent chemical behavior. Thallium(III) compounds and ions resemble those of boron. Thallium(I), the more common form, exhibits behaviour similar to that of the the alkali metals and is typically present in potassium ores. Commercially, the element is obtained as a gaseous byproduct of lead and zinc refining, or in the process of obtaining sulfuric acid from pyrites.

Applications for thallium are mostly limited to the fields of electronics, medicine, and infrared optics. Formerly, one of its primary commercial applications centered on thallium sulfate, a toxic, colorless and odorless substance used as a rodenticide; unfortunately, thallium sulfate is also highly toxic to humans, and a string of accidental poisonings led the United States to ban the substance in 1975 in favor of safer alternatives. Thallium in nearly all of its forms is toxic and potentially carcinogenic, and it must be handled with extreme care; the +1 oxidation state’s similarity to alkaline metals causes the ions to be taken up by the cellular potassium ion pumps. Despite this danger, the radioactive isotope thallium-201 is frequently used in medicine in scintigraphy-type stress tests to diagnose clogged arteries and heart disease. 201-Tl binds to heart tissue only with an adequate supply of blood and is quickly eliminated from the patient’s body, emitting gamma particles that can be detected with a scintillation detector and subsequently analysed to determine the robustness of the heart. Some thallium(III) salts are also used as reagents in laboratory science, and thallium hydroxide is a strong base used in the synthesis of organometallic compounds. Thallium iodide in combination with iodides of indium, sodium, scandium, dysprosium, and occasionally tin, are used to produce the high-efficiency white light of metal halide lamps. In addition, certain alloys of thallium are used in bearings, and contact points, and soluble thallium salts serve as additives to gold plating baths to improve the grain size and expedite the plating process.

Thallium has multiple uses in the field of advanced optoelectronics including infrared optics, phosphors, and specialty glasses. The high refractive index of the optical crystals thallium bromoiodide (KRS-5) and thallium bromochloride (KRS-6) make them ideal materials for attenuated total reflection prisms, lenses, and windows in infrared spectrometers and, like other thallium halides, can be used to manufacture fiber optics. Various phosphor and scintillator materials contain thallium either as in compound form, as in thallium selenide, or as host or activator, as in thallium-activated potassium chloride and thallium-activated sodium iodide. Photoelectric cells, photoresistors, and infrared detectors benefit from thallium sulfide’s ability to change electrical conductivity when exposed to infrared radiation; thallium chloride and thallium bromide have also been shown to be photosensitive. Thallium increases the refractive index of glasses; thallium(I) oxide is one material that has been used to fabricate optical glasses in combination with substances like germanium oxide and tellurium, and shatter-proof low melting glasses that become fluid at 125-150 degrees Celsius have been fabricated from thallium in combination with sulfur, selenium, or arsenic.

Thallium compounds have other special properties that give them unique uses. A eutectic thallium-mercury alloy with 8.5% thallium is exhibits a melting point 20 degrees below that of pure mercury, making it applicable in low-temperature thermometers. Strongly basic thallium hydroxide has the ability to take up carbon dioxide from the atmosphere,. Thallium is the only known impurity that causes lead telluride (PbTe) to exhibit superconductivity; the resulting thallium-substituted lead telluride, PbTe:Tl, is a semiconductor and thermoelectric material. Other superconducting materials like thallium barium calcium copper oxides (TBCCO) and various thallium cuprates exhibit T c as high as 127 K.

+ Open All
- Close All

Thallium is used most often in the form of thallium sulfide, a compound whose conductivity changes upon exposure to infrared light. High Purity (99.999%) Thallium Oxide (Tl2O) PowderThis property makes the compound useful in photocells. Thallium bromide-iodide crystals have been used as infrared optical materials. Thallium has also been used with sulfur, selenium High Purity (99.999%) Thallium (Tl) Sputtering Targetor arsenic to produce low melting glasses which become fluid between 125° and 150 C°.These glasses have properties at room temperatures similar to ordinary glasses and are said to be durable and insoluble in water. Thallium oxide has been used to produce glasses with a high index of refraction, and is also used in the manufacture of photo cells. Oxides are available in forms including powders and dense pellets for such uses as optical coating and thin film applications. Thallium is also available in many other specific states, forms and shapes including pellets, rod, wire and granules for evaporation source material purposes.

Thallium Properties

Thallium (Tl) atomic and molecular weight, atomic number and elemental symbolThallium is a post-transition metal that is not found free in nature. It is a Block P, Group 13, Period 6 element. Thallium Bohr ModelThe number of electrons in each of Thallium's shells is 2, 8, 18, 32, 18, 3 and its electron configuration is [Xe] 4f14 5d10 6s2 6p1. The Thallium atom has a radius of and its Van der Waals radius is Elemental ThalliumIn its elemental form, CAS 7440-28-0, thallium has a silvery white appearance. Thallium was first discovered by Sir William Crookes in 1861 and gets its name from the Greek word "thallos" which means twig or green shoot. Thallium is produced from trace amounts that are found in copper, lead, zinc, and other heavy-metal-sulfide ores. Thallium information, including technical data, safety data, high purity properties, research, applications and other useful facts are discussed below. Scientific facts such as the atomic structure, ionization energy, abundance on earth, conductivity and thermal properties are also included.

Symbol: Tl
Atomic Number: 81
Atomic Weight: 204.3833
Element Category: post-transition metal
Group, Period, Block: 13, 6, p
Color: silvery-gray
Other Names: N/A
Melting Point: 304 °C, 579 °F, 577 K
Boiling Point: 1473 °C, 2683 °F, 1746 K
Density: 11.85 g·cm3
Liquid Density @ Melting Point: 11.22 g·cm3
Density @ 20°C: 11.85 g/cm3
Density of Solid: 11850 kg·m3
Specific Heat: 129 J/(kg K)
Superconductivity Temperature: 2.38 [or -270.77 °C (-455.39 °F)] K
Triple Point: N/A
Critical Point: N/A
Heat of Fusion (kJ·mol-1): 4.31
Heat of Vaporization (kJ·mol-1): 166.1
Heat of Atomization (kJ·mol-1): 182.845
Thermal Conductivity: 46.1 W·m-1·K-1
Thermal Expansion: (25 °C) 29.9 µm·m-1·K-1
Electrical Resistivity: (20 °C) 0.18 µΩ·m
Tensile Strength: N/A
Molar Heat Capacity: 26.32 J·mol-1·K-1
Young's Modulus: 8 GPa
Shear Modulus: 2.8 GPa
Bulk Modulus: 43 GPa
Poisson Ratio: 0.45
Mohs Hardness: 1.2
Vickers Hardness: N/A
Brinell Hardness: 26.4 MPa
Speed of Sound: (20 °C) 818 m·s-1
Pauling Electronegativity: 1.62
Sanderson Electronegativity: 2.25
Allred Rochow Electronegativity: 1.44
Mulliken-Jaffe Electronegativity: 1.96 (sp2 orbital)
Allen Electronegativity: N/A
Pauling Electropositivity: 2.38
Reflectivity (%): N/A
Refractive Index: N/A
Electrons: 81
Protons: 81
Neutrons: 123
Electron Configuration: [Xe] 4f14 5d10 6s2 6p1
Atomic Radius: 170 pm
Atomic Radius,
non-bonded (Å):
Covalent Radius: 145±7 pm
Covalent Radius (Å): 1.44
Van der Waals Radius: 196 pm
Oxidation States: 3, 2, 1 (mildly basic oxide)
Phase: Solid
Crystal Structure: hexagonal close-packed
Magnetic Ordering: diamagnetic
Electron Affinity (kJ·mol-1) 36.375
1st Ionization Energy: 589.36 kJ·mol-1
2nd Ionization Energy: 1971.02 kJ·mol-1
3rd Ionization Energy: 2878.18 kJ·mol-1
CAS Number: 7440-28-0
EC Number: 231-138-1
MDL Number: MFCD00134063
Beilstein Number: N/A
SMILES Identifier: [Tl]
InChI Identifier: InChI=1S/Tl
PubChem CID: 5359464
ChemSpider ID: 4514293
Earth - Total: 3.86 ppb 
Mercury - Total: 0.044 ppb 
Venus - Total: 4.05 ppb
Earth - Seawater (Oceans), ppb by weight: 0.001
Earth - Seawater (Oceans), ppb by atoms: 0.00003
Earth -  Crust (Crustal Rocks), ppb by weight: 530
Earth -  Crust (Crustal Rocks), ppb by atoms: 50
Sun - Total, ppb by weight: 1
Sun - Total, ppb by atoms: 0.01
Stream, ppb by weight: N/A
Stream, ppb by atoms: N/A
Meterorite (Carbonaceous), ppb by weight: 80
Meterorite (Carbonaceous), ppb by atoms: 6
Typical Human Body, ppb by weight: N/A
Typical Human Body, ppb by atom: N/A
Universe, ppb by weight: 0.5
Universe, ppb by atom: 0.003
Discovered By: William Crookes
Discovery Date: 1861
First Isolation: Claude-Auguste Lamy (1862)

Health, Safety & Transportation Information for Thallium

Thallium and its compounds are highly toxic. Safety data for thallium metal, nanomaterials, and compounds can vary widely depending on the form. For potential hazard information, toxicity, and road, sea and air transportation limitations, such as DOT Hazard Class, DOT Number, EU Number, NFPA Health rating and RTECS Class, please see the specific Thallium material or compound referenced in the Products tab. The below information applies to elemental (metallic) Thallium.

Safety Data
Material Safety Data Sheet MSDS
Signal Word Danger
Hazard Statements H300-H330-H373-H413
Hazard Codes T+
Risk Codes 26/28-33-53
Safety Precautions 13-28-45-61
RTECS Number XG3425000
Transport Information UN 3288 6.1/PG 2
WGK Germany 3
Globally Harmonized System of
Classification and Labelling (GHS)
Skull and Crossbones-Acute Toxicity  Health Hazard

Thallium Isotopes

Thallium (Tl) has 37 isotopes with atomic masses ranging from 176 to 212. Only 203Tl and 205Tl are stable.

Nuclide Isotopic Mass Half-Life Mode of Decay Nuclear Spin Magnetic Moment Binding Energy (MeV) Natural Abundance
(% by atom)
176Tl 176.00059(21)# 5.2(+30-14) ms Unknown (3-,4-,5-) N/A 1330.97 -
177Tl 176.996427(27) 18(5) ms p to 176Hg (1/2+) N/A 1348.36 -
178Tl 177.99490(12)# 255(10) ms a to 174Au N/A N/A 1356.44 -
179Tl 178.99109(5) 270(30) ms a to 175Au (1/2+) N/A 1364.52 -
180Tl 179.98991(13)# 1.5(2) s a to 176Au; ß+ to 180Hg; SF N/A N/A 1381.91 -
181Tl 180.986257(10) 3.2(3) s a to 177Au; ß+ to 181Hg 1/2+# N/A 1389.99 -
182Tl 181.98567(8) 2.0(3) s ß+ to 182Hg; a to 178Au 2-# N/A 1398.07 -
183Tl 182.982193(10) 6.9(7) s ß+ to 183Hg; a to 179Au 1/2+# N/A 1406.15 -
184Tl 183.98187(5) 9.7(6) s ß+ to 184Hg 2-# N/A 1414.23 -
185Tl 184.97879(6) 19.5(5) s a to 181Au; ß+ to 185Hg 1/2+# N/A 1431.62 -
186Tl 185.97833(20) 40# s ß+ to 186Hg; a to 182Au (2-) N/A 1439.7 -
187Tl 186.975906(9) ~51 s ß+ to 187Hg (1/2+) N/A 1447.78 -
188Tl 187.97601(4) 71(2) s ß+ to 188Hg (2-) N/A 1455.86 -
189Tl 188.973588(12) 2.3(2) min ß+ to 189Hg (1/2+) N/A 1463.94 -
190Tl 189.97388(5) 2.6(3) min ß+ to 190Hg 2(-) N/A 1472.02 -
191Tl 190.971786(8) 20# min ß+ to 191Hg (1/2+) N/A 1480.09 -
192Tl 191.97223(3) 9.6(4) min ß+ to 192Hg (2-) N/A 1488.17 -
193Tl 192.97067(12) 21.6(8) min ß+ to 193Hg 1/2(+#) N/A 1496.25 -
194Tl 193.97120(15) 33.0(5) min ß+ to 194Hg; a to 190Au 2- N/A 1504.33 -
195Tl 194.969774(15) 1.16(5) h ß+ to 195Hg 1/2+ N/A 1521.73 -
196Tl 195.970481(13) 1.84(3) h ß+ to 196Hg 2- N/A 1520.49 -
197Tl 196.969575(18) 2.84(4) h ß+ to 197Hg; IT 1/2+ N/A 1537.88 -
198Tl 197.97048(9) 5.3(5) h ß+ to 198Hg 2- N/A 1536.65 -
199Tl 198.96988(3) 7.42(8) h ß+ to 199Hg; IT 1/2+ N/A 1554.04 -
200Tl 199.970963(6) 26.1(1) h EC to 200Hg 2- 0.04 1552.8 -
201Tl 200.970819(16) 72.912(17) h EC to 201Hg 1/2+ 1.605 1560.88 -
202Tl 201.972106(16) 12.23(2) d EC to 202Hg 2- 0.06 1568.96 -
203Tl 202.9723442(14) STABLE - 1/2+ 1.622257 1577.04 29.524
204Tl 203.9738635(13) 3.78(2) y EC to 204Hg; ß- to 204Pb 2- 0.09 1585.12 -
205Tl 204.9744275(14) STABLE - 1/2+ 1.6382135 1593.2 70.476
206Tl 205.9761103(15) 4.200(17) min ß- to 206Pb; IT 0- N/A 1601.28 -
207Tl 206.977419(6) 4.77(2) min ß- to 207Pb 1/2+ N/A 1609.35 -
208Tl 207.9820187(21) 3.053(4) min ß- to 208Pb 5(+) N/A 1608.12 -
209Tl 208.985359(8) 2.161(7) min ß- to 209Pb (1/2+) N/A 1616.2 -
210Tl 209.990074(12) 1.30(3) min ß- to 210Pb; ß- + n to 209Pb (5+)# N/A 1614.96 -
211Tl 210.99348(22)# 1# min [>300 ns] Unknown 1/2+# N/A 1623.04 -
212Tl 211.99823(32)# 30# s [>300 ns] Unknown 5+# N/A 1631.11 -