[Xe] 4f145d106s26p1
2, 8, 18, 32, 18, 3
304°C, 579°F, 577 K
1473°C, 2683°F, 1746 K
William Crookes
More Information
expand all +
collapse all -

Uses and Properties

Image Explanation

Originally employed as rat poison, Thallium was discontinued due to the associated risk of exposure.


A soft, silvery-white metal that tarnishes easily.


Thallium: Unveiling the Unique Uses of a Versatile Element

Thallium, a rare and distinctive element, has found applications across various industries, showcasing its versatility despite its initial use as a rat poison. From its discovery to contemporary uses in medicine and technology, Thallium continues to play a role in advancing science and industry. In this exploration, we delve into the diverse and unexpected applications of Thallium, revealing its significance beyond its notorious history.


1. Early Uses: A Lethal Rodenticide

Thallium's first notable application was as a rodenticide, where it was employed to control rat populations. The element's high toxicity made it effective in eliminating rodents, but its adverse effects on humans and other non-target species led to a reconsideration of its use. Thallium-based rat poisons have since been discontinued due to the associated health risks.


2. Medical Imaging: Thallium in Nuclear Medicine

One of the remarkable applications of Thallium lies in the field of nuclear medicine. Thallium-201, a radioisotope of Thallium, is utilized in myocardial perfusion imaging. This diagnostic technique helps assess blood flow to the heart muscle, aiding in the detection of coronary artery disease. Thallium-201 emits gamma rays, allowing for non-invasive imaging of the heart, providing valuable information for cardiologists and healthcare professionals.


3. Semiconductor Industry: Thallium Selenide in Electronics

Thallium compounds, particularly Thallium selenide (Tl2Se), have found applications in the semiconductor industry. Thallium selenide is a crucial component in infrared detectors and sensors. Its unique electronic properties make it well-suited for use in devices that detect infrared radiation, enabling applications in night vision technology and other infrared imaging systems.


4. Optical Lenses and Prisms: Thallium Bromide-Iodide Crystals

Thallium bromide-iodide crystals have optical properties that make them valuable in the manufacturing of lenses and prisms for infrared spectroscopy. These crystals are transparent in the infrared region of the electromagnetic spectrum, allowing them to be used in analytical instruments for chemical analysis. Thallium's contribution to optics extends beyond its early association with toxicity, demonstrating its potential in advancing scientific research.


5. Research in High-Temperature Supraconductors

Thallium-based high-temperature superconductors have become a subject of intense research in materials science. Thallium compounds, such as Thallium barium calcium copper oxide (TBCCO), exhibit superconductivity at relatively higher temperatures compared to conventional superconductors. This property holds promise for applications in magnetic levitation, advanced medical imaging, and energy transmission.


6. Historical Photography: Thallium in Photographic Toning

In the early days of photography, Thallium found use in the toning of black and white prints. Thallium compounds were applied to enhance the archival stability of photographic images. While this application is less prominent today due to environmental and health considerations, it reflects the historical role of Thallium in the evolution of photographic processes.


7. Environmental Monitoring: Thallium Analysis in Water and Soil

Thallium's toxicity has prompted its monitoring in environmental samples, particularly water and soil. Analytical techniques, such as atomic absorption spectroscopy, enable the detection and quantification of Thallium in trace amounts. This monitoring is crucial for assessing potential environmental contamination and ensuring the safety of ecosystems and human populations.


8. Glass Manufacturing: Thallium Oxide for Optical Glass

Thallium oxide is used in the manufacturing of optical glass with a high refractive index. This type of glass is employed in specialty lenses and prisms for applications like high-performance cameras and telescopes. Thallium's role in optical glass underscores its significance in precision instruments and imaging devices.


9. Catalysts in Organic Chemistry: Thallium(I) Compounds

Certain Thallium(I) compounds serve as catalysts in organic chemistry, facilitating specific chemical reactions. Thallium(I) acetate, for example, has been employed in the synthesis of organic compounds. While the use of Thallium compounds in catalysis is niche, it exemplifies the element's potential in driving advancements in synthetic chemistry.


10. Future Prospects: Thallium in Advanced Technologies

As research continues to unlock the properties and applications of Thallium, its potential in emerging technologies becomes increasingly evident. The element's unique electronic and superconducting properties hint at future applications in fields like quantum computing and advanced energy systems.



Thallium, once associated with its lethal history as a rodenticide, has transcended its notorious past to become a valuable element in diverse scientific and industrial applications. From its role in nuclear medicine to advancements in semiconductors and superconductors, Thallium's versatility continues to contribute to the progress of technology and scientific understanding. As we navigate the complexities of harnessing Thallium's potential, it serves as a reminder that elements, once considered hazardous, can evolve into essential contributors to our technological and scientific landscape.


The history of Thallium (Tl) is a captivating tale of discovery, exploration, and transformation. From its early identification as a mysterious element to its diverse applications across different eras, Thallium has left an indelible mark on science, industry, and even the darker realms of history. Join us as we unravel the story of Thallium, tracing its evolution from obscurity to prominence and exploring its pivotal moments in the annals of chemistry.


1. Discovery and Isolation

Thallium's journey began in 1861 when Sir William Crookes, an English chemist and physicist, discovered the element while studying residues from sulfuric acid production. Crookes noticed a distinctive green emission line in the spectroscopy analysis, indicating the presence of an unknown element. Subsequent isolation efforts led to the discovery of Thallium, named after the Greek word "thallos," meaning green shoot or twig, symbolizing its green spectral lines.


2. Early Alchemical Associations

Thallium's early interactions with humanity were somewhat ominous. Despite its discovery being a scientific triumph, its high toxicity became apparent, leading to its adoption as a rat poison in the late 19th century. This application, albeit effective in rodent control, marked a dark chapter in Thallium's history, as its toxicity posed risks to unintended targets and ecosystems.


3. Contributions to Photography

In the late 19th and early 20th centuries, Thallium found a more benign application in the realm of photography. Thallium compounds, particularly thallium sulfate, were employed in the toning process of black and white prints, enhancing their archival stability. This usage showcased Thallium's adaptability and willingness to contribute positively to emerging technologies.


4. Radioactive Isotopes and Medical Imaging

The mid-20th century saw a significant shift in Thallium's narrative, with the advent of nuclear medicine. Thallium-201, a radioactive isotope of Thallium, emerged as a crucial element in myocardial perfusion imaging. This diagnostic tool, used to assess blood flow to the heart muscle, revolutionized cardiac healthcare, illustrating Thallium's pivotal role in advancing medical diagnostics.


5. Semiconductor Industry Advancements

As technology progressed, Thallium found a new home in the semiconductor industry. Thallium selenide (Tl2Se), a compound of Thallium, exhibited unique electronic properties that proved beneficial for infrared detectors. The application of Thallium in this field demonstrated its versatility and ability to adapt to the demands of modern electronics.


6. Environmental Concerns and Regulatory Measures

Despite its contributions to various industries, Thallium's toxic nature did not go unnoticed. Environmental concerns and health risks associated with Thallium exposure led to regulatory measures, prompting a reconsideration of its applications. Rat poisons containing Thallium were banned, and efforts were made to minimize its impact on ecosystems.


7. Thallium in Advanced Materials

Thallium's unique properties, such as its superconducting abilities, attracted attention in the field of materials science. Thallium-based high-temperature superconductors, like Thallium barium calcium copper oxide (TBCCO), opened new possibilities for applications in magnetic levitation, advanced medical imaging, and energy transmission. The exploration of Thallium's potential in advanced materials continues to shape the landscape of scientific innovation.


8. Contemporary Applications and Future Prospects

In the 21st century, Thallium remains an element of intrigue and potential. Its applications in nuclear medicine, semiconductors, and advanced materials underscore its enduring relevance. Ongoing research aims to unlock further secrets of Thallium's properties, with an eye toward potential applications in emerging technologies such as quantum computing and beyond.



Thallium's history is a testament to the dual nature of chemical elements—capable of both peril and progress. From its mysterious discovery to its applications in photography, medicine, and materials science, Thallium has woven a narrative that reflects the evolution of scientific understanding and responsible technological use. As we navigate the complexities of harnessing Thallium's potential in the 21st century, its history serves as a reminder of the delicate balance between innovation and environmental stewardship in the realm of chemistry.

Atomic Data

Atomic Radiues, Non-bonded (A): 1.96
Electron Affinity (kJ mol-1): 36.375
Covalent Radiues (A): 1.44
Electronegativity (Pauling Scale): 1.8
Ionisation Energies (kJ mol-1) 1st 2nd 3rd 4th 5th 6th 7th 8th
589.351 1971.032 2878.16 - - - - -

Oxidation States and Isotopes

Common oxidation states 3, 1
Isotope Atomic Mass Natural Abundance Half Life Mode of Decay
203Tl 202.972 29.52 - -
205Tl 204.974 70.48 - -

Supply Risk

Relative Supply Risk: Unknown
Crustal Abundance (ppm): 0.85
Recycle Rate (%): Unknown
Production Conc.(%) : Unknown
Top 3 Producers:
Top 3 Reserve Holders:
Substitutability: Unknown
Political Stability of Top Producer: Unknown
Political Stability of Top Reserve Holder: Unknown

Pressure and Temperature Data

Specific Heat Capacity: 129
Shear Modulus: Unknown
Young Modulus: Unknown
Bulk Modulus: 43
Pressure 400k Pressure 600k Pressure 800k Pressure 1000k Pressure 1200k Pressure 1400k Pressure 1600k Pressure 1800k Pressure 2000k Pressure 2200k Pressure 2400k
- 1.59 x 10-5 0.0931 16.9 - - - - - - 43



Welcome back, dear listeners, to another exciting episode of "Talking About Elements," the podcast where we dive deep into the world of chemical elements. I'm your host, and today we're delving into the intriguing world of Thallium.

Thallium, symbol T L, and atomic number 81, is a remarkable element with a rich history, unique properties, various occurrences, and a wide range of applications. So, let's begin our journey through the fascinating world of thallium.

To understand the story of thallium, we must go back to the 19th century. Thallium was discovered in 1861 by Sir William Crookes, an English chemist. He detected the element spectroscopically while studying residues from sulfuric acid production. The name "thallium" originates from the Greek word "thallos," meaning "green twig," owing to the distinctive green spectral lines it emitted.

Thallium is a member of the periodic table's group 13 elements, often referred to as the "boron group." It's unique due to its soft, malleable nature, similar to lead, and it is sometimes called "the poisoner's poison" because of its high toxicity. In fact, it's one of the most toxic elements known to us.

Thallium is also notable for its interesting electronic configuration. It has a single valence electron, which makes it highly reactive. This property allows it to form various compounds, displaying a diverse range of chemical behavior.

Thallium is relatively rare in Earth's crust, with an average concentration of only about 0.5 parts per million. It is primarily found in association with other elements such as sulfur and mercury ores. Some of the notable minerals containing thallium include crooksite, hutchinsonite, and lorandite.

Extracting thallium from its ores is a complex process. One common method involves roasting the ore to convert thallium sulfide into thallium oxide, which is then reduced using carbon or hydrogen to obtain metallic thallium. However, due to its toxicity, handling thallium during its production requires extreme care and safety measures.

Now, let's explore the diverse applications of thallium. Despite its toxicity, this element has found its way into various fields.

Thallium is used in the manufacturing of semiconductors, photocells, and infrared detectors. Its electrical conductivity and photosensitivity make it valuable in these applications.

Thallium-based glass can transmit infrared radiation efficiently, making it useful for lenses and prisms in thermal imaging equipment.

Historically, thallium was used as a component in rat poisons and insecticides due to its toxicity. However, its use has been largely phased out for safety reasons. On a more positive note, thallium has been employed in medical imaging. Thallium-201 isotope is used in nuclear medicine for cardiac stress testing.

Thallium's dark history includes its use as a poison in various criminal cases. Its toxicity, combined with its subtle tastelessness and odorlessness, has made it a notorious choice for murder plots.

Thallium compounds are used in fundamental scientific research, particularly in the field of condensed matter physics, where they exhibit interesting electronic and magnetic properties.

As we've mentioned, thallium is highly toxic, and its presence in the environment can be detrimental. Accidental releases or improper disposal can lead to soil and water contamination, posing risks to ecosystems and human health. Thus, it's crucial to handle thallium with extreme care and responsibility.

Thallium, with its colorful history, unique properties, and wide range of applications, continues to intrigue scientists and enthusiasts alike. From its discovery in the 19th century to its role in modern technology and medicine, thallium remains a testament to the complexity and versatility of the chemical elements.

Thank you for joining us on this journey through the world of thallium. We hope you've enjoyed this episode of " Talking About Elements." Stay curious, and until next time, keep exploring the wonders of the periodic table. This is your host, signing off.


  • W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.

  • Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.

  • J. S. Coursey, D. J. Schwab, J. J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions (version 4.1), 2015, National Institute of Standards and Technology, Gaithersburg, MD, accessed November 2016.

  • T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.

  • John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.