Thallium is a very soft, malleable, heavy post-transition metal. It tarnishes in air, forms a toxic oxide, and is highly poisonous. Historically used in rat poisons, now mainly in electronics, optics, and special alloys.
The electron configuration of thallium is [Xe] 4f14 5d10 6s2 6p1. The presence of one electron in the 6p orbital explains its chemical reactivity and allows it to exhibit multiple oxidation states.
Thallium exhibits two main oxidation states: +1 and +3. The +1 oxidation state (in compounds like TlCl) is more stable due to the inert pair effect, where the 6s electrons resist bonding.
\(\mathrm{TlCl_3 \rightarrow TlCl + Cl_2}\)
This shows the reduction of thallium from +3 to +1 state.
Thallium is extremely toxic because it mimics potassium ions (K+) in biological systems, interfering with enzyme function and nerve signaling. Exposure can cause hair loss, nerve damage, and organ failure. Compounds like thallium sulfate (Tl2SO4) were once used as rat poison but are now banned in most countries.
Thallium is a soft, gray, heavy metal that can be easily cut with a knife. It has a melting point of 304 °C and a boiling point of 1473 °C. When exposed to air, it rapidly forms a dull gray oxide layer of thallium(I) oxide (Tl2O).
The inert pair effect in thallium occurs because the 6s electrons are tightly bound to the nucleus due to relativistic effects and poor shielding by inner d and f orbitals. As a result, they do not participate easily in bonding, making the +1 state more stable than +3.
Despite its toxicity, thallium is used in:
Common thallium compounds include:
Thallium reacts slowly with oxygen to form thallium(I) oxide:
\(\mathrm{4\,Tl + O_2 \rightarrow 2\,Tl_2O}\)
On further oxidation, thallium(III) oxide can form at higher temperatures.
Thallium behaves both like alkali metals and post-transition metals. In the +1 oxidation state, it forms ionic compounds similar to K+ or Na+, while in the +3 state, it forms covalent compounds such as TlCl3. Its chemistry bridges between s-block and p-block behavior.
No, thallium is not found free in nature. It occurs as a trace element in ores of copper, lead, and zinc, especially in the minerals crookesite (CuTlSe) and lorandite (TlAsS2). It is extracted as a by-product during smelting of these ores.