Thallium (Tl, atomic number 81) is a heavy metal known as one of the most toxic elements in the periodic table, often nicknamed “the poisoner’s poison.” It was discovered in 1861 via a brilliant green spectral line, which inspired its name from the Greek word thallos, meaning “green shoot.” Understanding the element’s physical nature helps explain its role in specialized technology and criminal history.
The Appearance of Pure Thallium
In its pure, metallic form, thallium appears bright, silvery-white or bluish-white, possessing a distinct metallic luster similar to tin. This initial appearance is fleeting due to the metal’s high reactivity with air.
The moment thallium is exposed to the atmosphere, it rapidly oxidizes, dulling its sheen almost immediately. This process forms a non-protective, heavy oxide crust on the surface, quickly changing the color to a dull bluish-gray. The element soon loses its metallic brilliance entirely, taking on a soft, lead-like appearance. To preserve its initial silvery state, pure thallium metal must be stored under oil or in an inert atmosphere.
Unique Physical Characteristics
Thallium is notably soft, often compared to lead or wax, and can be easily cut with a common knife at room temperature. This softness results from its unique electron configuration, which causes weak metallic bonding.
The metal has a high density, weighing approximately 11.8 grams per cubic centimeter. Thallium also has a relatively low melting point for a metal, becoming liquid at about 304 degrees Celsius (579 degrees Fahrenheit). This combination of softness and low melting temperature makes the element useful for specialized applications. Its high volatility contributes to the severe toxicity of many thallium compounds, allowing them to readily enter the environment.
Natural Sources and Applications
Thallium is found in trace amounts across the Earth’s crust, with an average concentration estimated at about 0.7 parts per million. Although it occurs naturally in minerals associated with potassium, commercial thallium is primarily recovered as a by-product during the smelting and refining of other metals, such as zinc, lead, and copper sulfide ores.
Historically, thallium compounds like thallium sulfate were notorious for use as potent, odorless, and tasteless rodenticides and insecticides. Due to the danger to humans and non-target animals, this application has been largely banned in many developed countries since the 1970s. Modern applications are highly specialized, relying on its unique properties in the electronics and optics industries.
Specialized Applications
Thallium compounds are used to manufacture specialized glasses, including those with a high index of refraction or those that melt at exceptionally low temperatures. Thallium sulfide is utilized in photocells because its electrical conductivity changes when exposed to infrared light, making it useful in infrared detection equipment. Additionally, the radioisotope Thallium-201 is used in nuclear cardiology for stress tests to assess blood flow to the heart muscle.