caption A small miniature of deep red cubes of galkhaite (one of the very few thallium-containing minerals and a very complex sulfosalt) on matrix from the famous Getchell Mine, Humboldt County, Nevada.. (Source: Carnegie Mellon University)

Thallium is a soft, bluish-white metallic element. Its atomic number is 81 and its symbol is Tl. It looks much like lead, but chemically is very similar to aluminum. It is so soft that it can be cut with a knife. It reacts easily with air, water, and most acids. It does not react violently like the alkali metals. Thallium was discovered in 1861 by the English chemist William Crookes.

Thallium and thallium compounds are very toxic, so some of their earlier uses (such as a rodent poison and an insecticide) have been discontinued. They can enter a body through the skin, by inhaling dust or fumes, and by direct ingestion. As a result, strict rules about the use of thallium and thallium compounds have been created by the U.S. Environmental Protection Agency (EPA).

Adding thallium to mercury lowers mercury’s freezing temperature, permitting its application in low-temperature thermometers.


Previous Element: Mercury

Next Element: Lead


Physical Properties
Color bluish-gray
Phase at Room Temp. solid
Density (g/cm3) 11.85
Hardness (Mohs) 1.2
Melting Point (K) 576.7
Boiling Point (K) 1730
Heat of Fusion (kJ/mol) 4.269
Heat of Vaporization (kJ/mol) 166
Heat of Atomization (kJ/mol) 182
Thermal Conductivity (J/m sec K) 46.1
Electrical Conductivity (1/mohm cm) 55.556
Source Zn/Pb smelting by-product
Atomic Properties
Electron Configuration [Xe]6s24f145d106p1
Number of Isotopes 55 (2 natural)
Electron Affinity (kJ/mol) 20
First Ionization Energy (kJ/mol) 589.4
Second Ionization Energy (kJ/mol) 1971
Third Ionization Energy (kJ/mol) 2878
Electronegativity 1.83
Polarizability (Å3) 7.6
Atomic Weight 204.38
Atomic Volume (cm3/mol) 17.2
Ionic Radius2- (pm) ---
Ionic Radius1- (pm) ---
Atomic Radius (pm) 170
Ionic Radius1+ (pm) 164
Ionic Radius2+ (pm) ---
Ionic Radius3+ (pm) 102.5
Common Oxidation Numbers +1, +3
Other Oxid. Numbers ---
In Earth's Crust (mg/kg) 8.50×10-1
In Earth's Ocean (mg/L) 1.90×10-5
In Human Body (%) 0%
Regulatory / Health
CAS Number 7440-28-0
OSHA Permissible Exposure Limit (PEL) No limits
OSHA PEL Vacated 1989 No limits
NIOSH Recommended Exposure Limit (REL) No limits
University of Wisconsin General Chemistry
Mineral Information Institute
Jefferson Accelerator Laboratory

When an element is burned, it creates a very specific spectrum of light. Thallium’s spectrum includes a distinctive bright green line. The name thallium comes from the Greek word thallios which means a green twig, which is a reference to this green line.


The thallium concentration in the Earth’s crust is 0.7 parts per million (ppm). It forms a small number of rare minerals, including crookesite and lorandite. These minerals form with the zinc mineral sphalerite. As a result, thallium is recovered as a by-product of processing zinc ores. It is also recovered from lead and copper ores, and from the dust that accumulates in the flues of the copper, zinc and lead smelters.

It is estimated that the thallium resources worldwide total approximately 17 million kilograms. These resources are found in Canada, Europe and the United States. As late as 1999, thallium was not recovered from ores in the U.S. Thallium is imported by the U.S. from Belgium, Mexico, Germany, and the United Kingdom.

In addition to these resources, approximately 630 million kilograms of thallium is contained in coal. As with other commodities, a way of recovering thallium from coal at a reasonable cost has not yet been developed.

Manganese nodules that form on the ocean floor contain thallium. However, it is still too expensive to gather these nodules, so they are presently not a source for thallium.


Thallium is used in a number of electronic devices. It is used in selenium rectifiers, gamma radiation detection equipment, and infrared radiation detection and transmission equipment.

It also has non-electrical uses. For example, thallium is added to glass to increase its density and refractive index (that is, its ability to break light into its component colors). It is also used as a catalyst to create certain organic compounds. Radioactive thallium compounds are used in medical applications.

As mentioned above, thallium is no longer used to make insecticides or for rodent control.

Substitutes and Alternative Sources

The supplies of thallium are more than enough to meet the demand for this element. As a result, there is presently no need to search for or to develop substitutes or alternative sources for thallium. Should these resources be used up, retrieving thallium from coal or from the deep ocean manganese nodules may one day become possible or even necessary.

Further Reading

Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Mineral Information Institute. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Mineral Information Institute should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.



Institute, M. (2008). Thallium. Retrieved from


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