Thallium
Definition and meaning of Thallium in chemistry.
Thallium is a chemical element with the symbol Tl and atomic number 81, recognized as a highly toxic, extremely soft, and malleable metallic element. Belonging to the boron group, this silvery-white post-transition metal tarnishes rapidly upon exposure to air to form a dark, dull oxide layer.
In more detail
Located in group 13 of the periodic table, thallium shares several physical properties with its neighbor lead, being soft enough to be easily cut with a knife and melting at a relatively low temperature. In its natural environment, it is not found as a free metal but occurs in trace amounts in various potassium-bearing minerals and is commercially recovered as a byproduct of the smelting of copper, zinc, and lead sulfide ores. Chemically, thallium is notable for exhibiting two primary oxidation states, +1 and +3, with the +1 state being significantly more stable due to the inert pair effect, which makes its 6s electrons reluctant to participate in bonding. Thallium compounds are notoriously toxic and were historically used as rodenticides and insecticides until their use was banned in many countries due to accidental human poisonings. The metal itself is highly reactive compared to other heavy metals, dissolving readily in nitric and sulfuric acids.
Key facts
| Field | General Chemistry |
|---|---|
| Symbol | Tl |
| Atomic number | 81 |
| Atomic mass | 204.38 u |
| Category | Post Transition Metal |
| State at room temperature | Solid |
| Melting point | 304 degrees Celsius |
| Toxicity | Extremely high, causes systemic poisoning |
| Year discovered | 1861 by Sir William Crookes |
Thallium is an essential component in the manufacturing of specialized infrared optical materials, as thallium bromoiodide crystals are highly transparent to infrared radiation and are used in thermal imaging lenses.
Frequently asked questions
Why was thallium known as the poisoner's poison?
Thallium salts are highly water-soluble, nearly tasteless, and odorless, making them extremely difficult to detect when mixed into food or drink during intentional poisonings.
What is the inert pair effect in thallium?
The inert pair effect occurs when the two s-orbital electrons in the outermost shell become tightly bound to the nucleus, causing thallium to favor the +1 oxidation state instead of the +3 state typical for its group.