Thallium Facts – Tl or Atomic Number 81

Thallium is a chemical element with the symbol Tl and atomic number 81. This soft, malleable metal is part of the post-transition metals group and is known for its high toxicity. Despite its dangerous nature, thallium is useful in electronics and medicine. Here is a look at thallium facts, from its discovery and properties to its uses and biological impact.

Discovery, Naming, and Isolation

Thallium was discovered in 1861 by the English chemist Sir William Crookes. He identified the element while conducting spectroscopic analysis of residues from sulfuric acid production. The name “thallium” comes from the Greek word thallos, meaning “a green shoot or twig,” referring to the bright green spectral line that Crookes observed during his analysis.

Crookes initially isolated thallium in its oxide form. The purified metal was isolated in 1862 by the French chemist Claude-Auguste Lamy, who reduced thallium oxide with hydrogen. This marked the first time that pure thallium was obtained, revealing its soft, malleable nature and metallic luster.

Allotropes, Appearance, and Properties

Thallium is a dense metal that is soft enough that it easily cuts with a knife. The metal has a silvery-gray appearance, but it quickly tarnishes in air, forming a bluish-gray oxide layer that makes the metal resemble lead. Thallium is both malleable and ductile. The element has moderate electrical conductivity, but low thermal conductivity for a metal. The element forms various compounds, including halides, oxides, sulfides, and organometallic compounds.

Element Group

Thallium is a post-transition metal in group 13 of the periodic table, along with boron, aluminum, gallium, and indium. Thallium’s chemistry is somewhat similar to that of the other elements in this group, particularly lead and mercury, but its toxicological profile is much more severe. However, its common +1 oxidation state also gives the element some similarities to the alkali metals of group 1.

Isotopes

In natural, thallium consists almost entirely of two stable isotopes: Tl-203 (29.52% natural abundance) and Tl-205 (70.48% natural abundance). In addition to these stable isotopes, the element also naturally occurs as five short-lived radioisotopes that result from the decay processes of other elements. Including the synthetic isotopes, the element has 41 isotopes, with mass numbers ranging from 176 to 216. The synthetic isotopes mainly come from nuclear reactors or particle accelerators and have applications in scientific research and medical diagnostics.

Abundance and Sources

Thallium is a relatively rare element in the Earth’s crust, with an average concentration of about 0.7 mg/kg. It does not occur in its free state in nature. Sources include minerals such as crookesite (CuThSe), lorandite (TlAsS₂), and hutchinsonite ((Pb, Tl)₂As₅S₉). Thallium is also a trace element in some sulfide ores of zinc, copper, and lead.

Commercially, thallium extraction is a byproduct during the smelting of these sulfide ores. The metal is isolated from the dust and gases produced during the roasting of pyrites in the sulfuric acid manufacturing process.

Purification

The purification of thallium involves several steps. The initial step is dissolving crude thallium from smelting into acid, typically sulfuric acid, forming a solution of thallium sulfate. The thallium precipitates out of this solution as thallium(I) chloride by the addition of hydrochloric acid. Finally, the purified metal is obtained by reducing the thallium chloride with a suitable reducing agent, such as zinc or iron.

Uses of Thallium

Past Applications

• Rat Poison and Insecticides: Thallium compounds, particularly thallium sulfate, were popular rat poisons and insecticides due to their high toxicity. However, these uses have been largely discontinued due to the dangers posed to humans and other non-target species.
• Pharmaceuticals: In the past, thallium was useful for treating ringworm, skin infections, and sweating from tuberculosis. However, the severe side effects and high toxicity led to the discontinuation of these applications.

Present Applications

• Electronics: Thallium is used in the electronics industry, particularly in the manufacture of semiconductors and low-temperature thermometers. Thallium-based semiconductors are used in some infrared detectors.
• Medical Imaging: Thallium-201 is a radioactive isotope used in nuclear medicine, particularly in stress tests for heart disease. It is a tracer for assessing blood flow to the heart.
• Superconductors: Thallium is a component in certain high-temperature superconductors, particularly in cuprate superconductors, which have critical temperatures above the boiling point of liquid nitrogen.
• Glass Manufacturing: Thallium is used in the manufacture of special glasses, particularly those that are highly refractive, infrared, or useful in fiber optics.
• Pesticides: While largely banned, some countries still use thallium compounds in limited applications as rodenticides and insecticides.
• Eutectics: A eutectic alloy of mercury and thallium freezes at −60 °C, which is well below the freezing point of mercury. This alloy finds use in low-temperature switches and thermometers.

Other uses include organic synthesis, Clerici solution, seawater batteries, gold plating, and metal-halide lamps.

Oxidation States

Thallium commonly exhibits oxidation states of +1 and +3, with the +1 state being more stable. The +3 state is more oxidizing and less stable, especially in aqueous solutions, where it reduces to the +1 state. The +1 oxidation state occurs in compounds like thallium chloride (TlCl), whereas the +3 state is seen in compounds like thallium trioxide (Tl₂O₃). Less common oxidation states are -5, -2, -1, and +2.

Biological Role, Health Effects, and Toxicity

Thallium has no known biological role in humans or other organisms and is highly toxic. It interferes with several cellular processes, particularly potassium transport, which cause severe symptoms, including hair loss, nerve damage, and organ failure.

In Humans:

Thallium and its compounds are odorless and tasteless. The +1 oxidation state readily passes into the bloodstream through skin, where it replaces potassium. The +3 oxidation state behaves more like a heavy metal. Thallium salts have a notorious reputation as a poison, either intentional or accidental. Removing it from the body is difficult, but oral administration of Prussian blue, hemodialysis, hemoperfusion, and administration of potassium are useful treatments. The National Institute for Occupational Safety and Health (NIOSH) sets a recommended exposure limit (REL) for skin exposure to 0.1 mg/m² for an eight-hour day and 0.1 mg/m² in an eight-hour time-weighted average for inhalation.

• Acute Exposure: Symptoms of acute thallium poisoning include nausea, vomiting, diarrhea, and abdominal pain. Neurological symptoms include peripheral neuropathy, tremors, and seizures.
• Chronic Exposure: Long-term exposure causes alopecia (hair loss), skin lesions, and damage to the liver, kidneys, and nervous system. While poisonous, thallium is not a suspected carcinogen.

Sources of thallium pollution include coal-burning power plants, metal sewers, ore processing operations, and the emissions from cement factories.

In Other Organisms:

Thallium is toxic to both animals and plants. It accumulates in the food chain, leading to secondary poisoning in predatory species. Thallium’s interference with potassium channels makes it particularly dangerous to plants, where it inhibits growth and photosynthesis.

Key Thallium Facts for Scientists

Here is a summary of essential thallium facts, including the element’s atomic and physical data:

References

• Crookes, William (April 1861). “XLVI. On the existence of a new element, probably of the sulphur group”. Philosophical Magazine. 21 (140): 301–305. doi:10.1080/14786446108643058
• Hasan, Heather (2009). The Boron Elements: Boron, Aluminum, Gallium, Indium, Thallium. Rosen Publishing Group. ISBN 978-1-4358-5333-1.
• Lamy, Claude-Auguste (1862). “De l’existencè d’un nouveau métal, le thallium”. Comptes Rendus. 54: 1255–1262.
• Peter, A.; Viraraghavan, T. (2005). “Thallium: a review of public health and environmental concerns”. Environment International. 31 (4): 493–501. doi:10.1016/j.envint.2004.09.003
• Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. ISBN 0-8493-0464-4.