Thallium
Definition and meaning of Thallium in chemistry.
![Thallium element card: symbol Tl, atomic number 81, atomic mass 204.38, electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹, oxidation states +3 +1, Post-transition metal, group 13, period 6, solid at room temperature.](/assets/images/terms/thallium-chemical-element.webp)
Thallium is a highly toxic and extremely soft metallic element. It has the chemical symbol Tl and the atomic number 81. This silvery-white post-transition metal tarnishes rapidly when exposed to air. The reaction forms a dark and dull oxide layer on its surface.
In more detail
Thallium sits in group 13 of the periodic table alongside boron and aluminum. It shares several physical properties with its heavy neighbor lead. The metal melts at a relatively low temperature.
It is also soft enough that a person can easily cut it with a knife. Thallium does not exist as a free metal in the natural environment. Instead, it occurs in very small amounts within various potassium-bearing minerals.
Companies recover it as a byproduct when melting, or smelting, copper, zinc, and lead sulfide ores. Chemically, thallium exhibits two primary oxidation states of +1 and +3. The +1 state remains significantly more stable due to the inert pair effect.
This effect happens when the two outer 6s electrons resist participating in chemical bonds. Thallium compounds are notoriously toxic and can cause severe systemic poisoning in humans. People historically used them as powerful rat poisons and agricultural insecticides.
Many countries eventually banned these uses due to frequent accidental poisonings. The pure metal is highly reactive compared to other heavy metals like gold. It dissolves rapidly when placed in strong nitric and sulfuric acids. When this happens, it forms soluble thallium salts that easily enter water supplies.
Key facts
| Field | General Chemistry |
|---|---|
| Symbol | Tl |
| Atomic number | 81 |
| Atomic mass | 204.38 u |
| Category | Post-Transition-Metal |
| Melting point | 304 degrees Celsius |
| Toxicity | Extremely high, causes systemic poisoning |
Thallium is an essential component in the manufacturing of specialized infrared optical materials. Engineers use thallium bromoiodide crystals to build advanced thermal imaging lenses. These specific crystals are highly transparent to invisible infrared radiation.
Frequently asked questions
Why was thallium known as the poisoner's poison?
Thallium salts dissolve easily in water and remain nearly tasteless and odorless. This made them extremely difficult to detect in food or drink. Criminals often used them during intentional poisonings for this exact reason.
What is the inert pair effect in thallium?
The inert pair effect happens when the two s-orbital electrons bind tightly to the nucleus. They refuse to share themselves during chemical reactions. This causes thallium to favor the +1 oxidation state instead of the +3 state.
Related terms
Sources & references
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