Thallium is a chemical element with the symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861, in residues of sulfuric acid production. Both used the newly developed method of flame spectroscopy, in which thallium produces a notable green spectral line. Thallium, from Greek θαλλός, thallós, meaning "green shoot" or "twig", was named by Crookes. It was isolated by both Lamy and Crookes in 1862; Lamy by electrolysis, and Crookes by precipitation and melting of the resultant powder. Crookes exhibited it as a powder precipitated by zinc at the international exhibition, which opened on 1 May that year. Thallium tends to form the +3 and +1 oxidation states. The +3 state resembles that of the other elements in group 13 (boron, aluminium, gallium, indium). However, the +1 state, which is far more prominent in thallium than the elements above it, recalls the chemistry of alkali metals, and thallium(I) ions are found geologically mostly in potassium-based ores, and (when ingested) are handled in many ways like potassium ions (K+) by ion pumps in living cells.

Commercially, thallium is produced not from potassium ores, but as a byproduct from refining of heavy-metal sulfide ores. Approximately 65% of thallium production is used in the electronics industry, and the remainder is used in the pharmaceutical industry and in glass manufacturing. It is also used in infrared detectors. The radioisotope thallium-201 (as the soluble chloride TlCl) is used in small amounts as an agent in a nuclear medicine scan, during one type of nuclear cardiac stress test.

Soluble thallium salts (many of which are nearly tasteless) are highly toxic, and they were historically used in rat poisons and insecticides. Because of their nonselective toxicity, use of these compounds has been restricted or banned in many countries. Thallium poisoning usually results in hair loss. Because of its historic popularity as a murder weapon, thallium has gained notoriety as "the poisoner's poison" and "inheritance powder" (alongside arsenic).

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Atomic properties

Standard atomic weight204.38 ±0.01 [204.382 … 204.385]
Atomic mass204.382 u

Atomic radii

Radius (empirical)170 pm
Radius (calculated)156 pm
Covalent radius145 ±7 pm
Van der Waals radius196 pm

Atomic shell

Electron configurationXe 4f14 5d10 6s2 6p1
Ionization energy(1st) 6.1082873 eV
(2nd) 20.4283 eV
(3rd) 29.852 eV
(4th) 51.14 eV
(5th) 62.6 eV
Shell model

Physical properties

Density11.85 g·cm−3
Molar volume1.722·10-5 m3·mol−1
Speed of sound818 m·s−1 (293.1 K)


Melting point577 K
Boiling point1,746 K
Liquid range1,169 K
Transition temperature2.4 K


Melting enthalpy4.2 kJ·mol-1
Enthalpy of vaporization165 kJ·mol-1
Binding energy182 kJ·mol-1

Heat and conductivity

Specific heat capacity129 J·kg−1·K−1
Thermal conductivity46 W·m-1·K-1
Expansion coefficient2.99·10-5 K-1


Mohs hardness1.2
Brinell hardness26.4 NM·m-2

Elastic properties

Young’s modulus8 GPa
Shear modulus2.8 GPa
Bulk modulus43 GPa
Poisson’s ratio0.45

Electrical properties

Electrical conductivity6.67·106 S·m-1
Resistance1.5·10-7 Ωm


Magnetic susceptibility-5.09·10-5 cm3·mol−1 (298 K)

Chemical properties

Basicityslightly basic
Oxidation state1, 3
Standard potential-0.3363 V (Tl+ + e- → Tl)


Pauling scale1.62
Sanderson scale2.25
Allred-Rochow scale1.44
Mulliken scale1.96
Nagle scale1.59
Pearson absolute negativity3.2 eV

Other properties

Natural occurrenceprimordial
Crystal structureHexagonal close-packed
Goldschmidt Classificationchalcophile
Superconductorwith transition tempperature (solid body, normal pressure)
Price/kg4,200 USD

Natural abundances

0.5 ppb ≈ 5.02·109 M☉
1 ppb ≈ 1.98·109 Mt
80 ppb ≈ 80 mg
Earth’s crust
530 ppb ≈ 14,600 Mt
0.001 ppb ≈ 1.37 kt