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Thallium,  81Tl
General properties
Name, symbol Thallium, Tl
Appearance Silvery white metal
Thallium in the periodic table


Atomic number 81
Standard atomic weight (Ar) 204.38
Group, block 13; p-block
Period period 6
Electron configuration [Xe] 4f14 5d10 6s2 6p1
per shell
2, 8, 18, 32, 18, 3
Physical properties
Phase Solid
Melting point 577 K ​(304 °C, ​579 °F)
Boiling point 1746 K ​(1473 °C, ​​2683 °F)
Density near r.t. 11.85 g/cm3
when liquid, at  11.22 g/cm3
Heat of fusion 4.14 kJ/mol
Heat of 165 kJ/mol
Molar heat capacity 26.32 J/(mol·K)
Atomic properties
Oxidation states 3, 2, 1, −1, −2, −5 ​ ​(a mildly basic oxide)
Electronegativity Pauling scale: 1.62
energies 1st: 589.4 kJ/mol
2nd: 1971 kJ/mol
3rd: 2878 kJ/mol
Atomic radius empirical: 170 pm
Covalent radius 145±7 pm
Van der Waals radius 196 pm
Crystal structure ​​hexagonal close-packed (hcp)
Speed of sound thin rod 818 m/s (at 20 °C)
Thermal expansion 29.9 µm/(m·K) (at 25 °C)
Thermal conductivity 46.1 W/(m·K)
Electrical resistivity 0.18 Ω·m (at 20 °C)
Magnetic ordering Diamagnetic
Young's modulus 8 GPa
Shear modulus 2.8 GPa
Bulk modulus 43 GPa
Poisson ratio 0.45
Mohs hardness 1.2
Brinell hardness 26.5–44.7 MPa
CAS Registry Number 7440-28-0
Discovery William Crookes (1861)
First isolation Claude-Auguste Lamy (1862)
· references

Thallium is a chemical element with symbol Tl and atomic number 81. It is a toxic element with few common applications.



Thallium quickly oxidizes in air to form a layer of thallium oxide. Acids, such as sulfuric and nitric acids, will dissolve thallium forming their respective salts. However, thallium is largely unaffected by hydrochloric acid, due to an insoluble layer of thallium(I) chloride.

While thallium displays 3+ oxidation state, like the elements from its group (aluminium, gallium) the most common oxidation state encountered for thallium is the +1 oxidation state. This state gives thallium several properties similar to that of alkali metals.


Thallium is extremely soft, malleable metal which can be cut cut with a knife at room temperature. It has a metallic luster that, when exposed to air, quickly tarnishes to a bluish-gray tinge, resembling lead. Thallium melts at 304 °C and boils at 1473 °C. It is quite dense, having a density of 11.85 g/cm3, slightly denser than lead.


Thallium is sold by various chemical suppliers, though it may not be cheap due to its hazardous nature. In many places the sale of thallium metal and compounds is restricted.

Certain old rat poisons may contain thallium compounds.


Thallium can be extracted from its compounds by reducing them, however this process is very dangerous, as thallium is extremely toxic, fact accentuated by its low melting point.




Thallium is extremely toxic and most of its compounds pose severe toxicity and is also a potent carcinogenic. Most thallium salts are nearly tasteless, which makes accidental poisoning very easy. People can be exposed to thallium by breathing it in, skin absorption, swallowing it, or eye contact. One of the most distinct effects of thallium poisoning is hair loss and fingernail peeling (Mees' lines).


Thallium is best stored in ampoules or under a liquid, like mineral oil.


Thallium and its compounds must be taken to special disposal facilities.


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