QUICK REFERENCE
- Symbol: Tl
- Atomic Number: 81
- Atomic Weight: 204.38
- Element Classification: Post-transition Metal
- Discovered By: Sir William Crookes
- Discovery Date: 1861
- Name Origin: From the Greek ‘thallos’, meaning green twig or shoot, referring to the bright green spectral line that indicated its presence
- Density(g/cc): 11.85
- Melting Point: 304°C
- Boiling Point: 1473°C
- Appearance: Soft, malleable, gray metal
- Atomic Radius(pm): 156
Discovery
Thallium was discovered by British chemist Sir William Crookes in 1861 while he was conducting flame spectroscopy experiments on selenium compounds. He observed a bright green spectral line that he correctly attributed to a new element. The element was isolated in greater quantity and studied further by the French chemist Claude-Auguste Lamy around the same time. The name “thallium” is derived from ‘thallos’, a Greek word meaning green twig, which Crookes chose to describe the green spectral line that led to its discovery.
Relation to Other Elements
Thallium is a post-transition metal, located in group 13 of the periodic table, sharing the group with boron, aluminum, gallium, and indium. Thallium exhibits properties of both metals and metalloids. It has similarities to lead in terms of its softness and high density. Thallium typically forms monovalent (Tl+) compounds, reminiscent of the alkali metals, but can also form trivalent (Tl3+) compounds similar to its group 13 counterparts.
Natural Occurrence
Thallium is found in the Earth’s crust at low concentrations, occurring naturally in sulfide ores such as pyrite, and is often extracted as a by-product of zinc and lead smelting. Thallium minerals are rare, with lorandite (TlAsS2) and hutchinsonite (TlPbAs5S9) being among the few thallium-specific minerals.
Uses
Thallium has several applications, though its use has been restricted due to its toxicity:
- Electronics: Thallium sulfide’s electrical conductivity changes with exposure to infrared light, making it useful in photoresistors.
- Glass Manufacturing: Thallium oxide is used to produce glasses with a high refractive index, suitable for optical lenses.
- Medical: Radioactive thallium isotopes are used in nuclear medicine, particularly in stress tests to diagnose coronary artery disease.
- Pest Control and Rodenticides: Historically, thallium sulfate was used as a rodenticide and ant killer, but its use has been largely discontinued due to safety concerns.
The discovery of thallium expanded the understanding of element behavior, especially in relation to spectroscopy, a fundamental tool in chemical analysis. Due to its toxicity, the handling and use of thallium require strict safety precautions, and its applications are limited and closely regulated.