THALLIUM

Name: Thallium (from the Greek "Thallos", literally "stem" in allusion to the green color of the flame)

Symbol: Tl

Group: 13

Period: 6

Block: p

Category: Post-transition metals

Atomic number: 81

Atomic mass: 204.38 u

Electrons per shell: 2, 8, 18, 32, 18, 3

Electronegativity: 1.62

Density: 11.85 g/cc

Melting point: 304ºC

Boiling point: 1473ºC

Thermal conductivity: 46 W (m·K)

Electrical conductivity: 6.7 × 10^6 S/m

Magnetic order: Diamagnetic

Ordinary state: Solid

Oxidation States: +1, +3

Mohs Hardness: 1.2

Vickers Hardness: No data

Brinell Hardness: 26.4 MPa

Most stable isotopes: Tl-203 (29.5%), Tl-205 (70.5%)

Discoverer: Claude-Auguste Lemy, French (1862)

Thallium (Tl), a chemical element with atomic number 81, is a post-transition metal of Group 13, discovered in 1861 by British chemist William Crookes using flame spectroscopy. However, its isolation in 1862 is attributed to both Crookes and the Frenchman Claude-Auguste Lamy, who worked independently. With an abundance of 0.85 ppm in the Earth's crust, thallium is a rare metal, scarcer than gold (Au), and its high cost (€48 per 100 g) and toxicity limit its applications. Its name derives from the Greek thallos (“green shoot” or “branch”), referring to the bright green line observed in its emission spectrum, a nod to its characteristic color that resonates with the Spanish word tallo (stem). It should not be confused with thulium (Tm), a lanthanide.

The discovery of thallium occurred in the context of the rise of spectroscopy, following the advances by Robert Bunsen and Gustav Kirchhoff in the 1860s. Crookes, analyzing selenium residues from a sulfuric acid factory, identified an unknown green line in 1861, publishing his finding in Chemical News. In 1862, he isolated small quantities of the metal and studied its compounds. In parallel, Lamy, in Lille, France, analyzed pyrite deposits from sulfuric acid production and isolated 14 g of thallium using a spectroscope borrowed from his brother-in-law, Jules Frédéric Kuhlmann. Although Crookes named the element, Lamy is recognized as a co-discoverer for his simultaneous isolation, a fact highlighted at the International Exhibition of London in 1862, where both presented samples of the metal.

Historically, thallium had a low profile due to its reactivity and extreme toxicity, comparable to arsenic (As). Its most infamous use was as a poison, especially in the form of thallium sulfate (Tl₂SO₄), which is odorless and tasteless. It was used as a rodenticide and, in documented cases, for assassinations, such as in Agatha Christie's novel The Pale Horse. Its toxicity, which causes alopecia and neurological damage, led to its prohibition as a pesticide in the United States in 1972 and in other countries thereafter. Although "boring" in terms of popularity and with limited applications in optics (thallium oxide, Tl₂O₃, for lenses) and electronics (thallium sulfide, Tl₂S, in photoresistors), thallium remains relevant in medical imaging (isotope Tl-201) and infrared detectors. Its reputation as the "ugly duckling" of Group 13 reflects its rarity, cost, and danger, but its discovery marked a milestone in chemical spectroscopy.

Thallium (Tl), a chemical element with atomic number 81, is a post-transition metal of Group 13 (pnictogens), known for its high toxicity, reactivity, and scarcity. With a density of 11.85 g/cm³ and an abundance of 0.85 ppm in the Earth's crust, thallium is rarer than lead (Pb) and significantly more expensive (€48 per 100 g in 2025). Its opaque grayish appearance, which quickly fades due to oxidation, lacks the luster of noble metals, and its mechanical fragility (Mohs hardness of ~1.2) makes it unsuitable for structural applications. Although it shares certain superficial similarities with lead, thallium does not possess the versatile properties that justify the popularity of lead, often being described as a metal with more disadvantages than virtues.

Physically, thallium is soft and malleable, capable of being cut with a knife, but its low mechanical strength limits its practical use. Its melting point is moderate (304 °C), higher than mercury (Hg, -38.83 °C) but lower than lead (327.5 °C), and its boiling point is 1,473 °C. Its electrical conductivity (6.7 × 10⁵ S/m) and thermal conductivity (46 W/(m·K)) are modest, inferior to those of metals like copper (Cu). Chemically, thallium is highly reactive for a post-transition metal, with a predominant oxidation state of +1 (instead of the +3 common in Group 13), which makes it similar to alkali metals, such as potassium (K). This characteristic, known as the inert pair effect, results from the relativistic stabilization of the 6s electrons, causing thallium to form compounds like thallium sulfate (Tl₂SO₄) or thallium oxide (Tl₂O), similar to those of alkali metals.

Thallium's reactivity and toxicity distinguish it negatively. Its chemical similarity to potassium allows it to replace potassium in biological processes, a phenomenon of ionic mimicry that makes it extremely dangerous. In the human body, thallium interferes with enzymatic functions, causing neurological and kidney damage, and alopecia, and is classified as a probable carcinogen by the International Agency for Research on Cancer (IARC). It is found in minerals like lorandite (TlAsS₂) or as an impurity in zinc and lead ores, often substituting for potassium. Although its use is limited due to its toxicity, thallium is employed in optics (thallium oxide for high-refraction lenses), electronics (thallium sulfide in photoresistors), and nuclear medicine (Tl-201 isotope for cardiac studies). Its scarcity, reactivity, and dangerousness make it a niche material, handled with strict precautions in specialized applications.

Thallium (Tl), a chemical element with atomic number 81, is a post-transition metal of Group 13, characterized by its high reactivity, toxicity, and density (11.85 g/cm³). With an abundance of 0.85 ppm in the Earth's crust, it is a rare and expensive metal (€48 per 100 g in 2025), more reactive than other p-block metals like indium (In) or tin (Sn). Its predominant oxidation state (+1), influenced by the inert pair effect, chemically resembles alkali metals such as potassium (K), rather than its Group 13 counterparts, which contributes to its instability against corrosion. This reactivity, combined with its toxicity, severely limits its applications, relegating it to specialized uses.

Thallium is highly reactive and does not resist prolonged exposure to air, even dry air. In dry or humid air, it rapidly forms thallium oxide (Tl₂O) or thallium hydroxide (TlOH), leading to advanced corrosion that dulls its initial grayish surface. To protect it, thallium must be stored submerged in mineral oil or in inert atmospheres, such as argon (Ar), to prevent oxidation. In fresh or salt water, thallium readily dissolves, forming soluble compounds like thallium chloride (TlCl) in the presence of chlorides. It is attacked by almost all acids, such as nitric acid (HNO₃), which forms thallium nitrate (TlNO₃), and sulfuric acid (H₂SO₄), which produces thallium sulfate (Tl₂SO₄), both highly toxic. However, hydrochloric acid (HCl) is an exception, as it generates a passivating layer of thallium chloride (TlCl), which retards corrosion. Interestingly, this compound, although derived from a toxic metal, is considered less dangerous and is used in nuclear medicine, particularly with the Tl-201 isotope, to evaluate cardiovascular function through myocardial perfusion imaging.

Thallium is a heavy metal both due to its density, superior to lead (Pb, 11.34 g/cm³), and its extreme toxicity, surpassing lead and bismuth (Bi), which is significantly less dangerous. Its ability to mimic potassium in biological processes (ionic mimicry) makes it particularly harmful, causing neurological and kidney damage, and alopecia. Despite its reactivity, thallium's properties do not make it more competitive than other p-block metals, such as indium or gallium (Ga), even in the absence of toxicity, due to its mechanical fragility (Mohs hardness ~1.2) and high cost. Its use is limited to specific applications, such as high-refraction lenses (thallium oxide, Tl₂O₃) and photoresistors (thallium sulfide, Tl₂S), where its reactivity and toxicity require strict handling in controlled environments.

Thallium (Tl), a chemical element with atomic number 81, is a post-transition metal of Group 13, with a density of 11.85 g/cm³ and an abundance of 0.85 ppm in the Earth's crust. Its high toxicity, reactivity, and cost (€48 per 100 g in 2025) limit its use in metallurgy, where it is not deliberately incorporated into alloys. When found in metals like lead (Pb), it is usually present as an unintended trace, derived from shared minerals such as galena (PbS). Classified as a heavy metal due to its density and extreme toxicity, thallium is less versatile than other p-block metals like indium (In) or tin (Sn). However, its compounds, rather than the elemental metal, have had historical and specialized applications, particularly in medicine, optics, and, infamously, as a poison.

In metallurgy, thallium has no significant applications due to its mechanical fragility (Mohs hardness ~1.2) and reactivity. Its presence in alloys is accidental, often as an impurity in lead or zinc (Zn), and its toxicity discourages intentional use. In contrast, thallium compounds have been historically relevant. In the past, salts such as thallium sulfate (Tl₂SO₄) were used as rodenticides and pesticides due to their odorless and tasteless nature, which made them effective but dangerous. This characteristic made them a poison of choice in criminal cases, as they could be mixed into food or drinks without detection, causing mass poisoning or selective assassinations, as documented in historical cases and in literature, such as Agatha Christie's The Pale Horse. Thallium's toxicity, which causes alopecia, neurological, and kidney damage, led to its prohibition as a pesticide in many countries, such as the United States in 1972, due to its impact on health and the environment.

In medicine, thallium compounds have had paradoxical applications. Historically, low doses of thallium salts were used to treat skin conditions, such as seborrheic dermatitis (known as "empeine" in some contexts), and diseases like tuberculosis or night sweats. However, its toxicity limited these uses, and today they have been replaced by safer alternatives. The radioactive isotope Tl-201 is used in nuclear cardiology to evaluate myocardial perfusion, detecting possible coronary heart diseases. Although effective, its use has decreased in favor of technetium-99m (Tc-99m), a less toxic and more practical isotope. In optics, thallium oxide (Tl₂O₃) is used in high-refraction lenses, and in electronics, thallium sulfide (Tl₂S) is employed in photoresistors for infrared detectors, leveraging its light sensitivity. Thallium's toxicity, comparable to arsenic (As), and its chemical reactivity, similar to alkali metals due to its +1 oxidation state, require extremely careful handling. Its global production (~10 metric tons annually in 2025) is limited, and its use is concentrated in highly specialized applications where its optical or radioactive properties are irreplaceable. Although thallium does not rival the versatility of other metals, its history as a poison and its role in medical and technological applications reflect a complex legacy, marked by the need to balance its benefits with its inherent risks.