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81 mercurythalliumlead


Name, Symbol, Number thallium, Tl, 81
Chemical series poor metals
Group, Period, Block 13, 6, p
Appearance silvery white
Standard atomic weight 204.3833(2)  g·mol−1
Electron configuration [Xe] 4f14 5d10 6s2 6p1
Electrons per shell 2, 8, 18, 32, 18, 3
Physical properties
Phase solid
Density (near r.t.) 11.85  g·cm−3
Liquid density at m.p. 11.22  g·cm−3
Melting point 577 K
(304 °C, 579 °F)
Boiling point 1746 K
(1473 °C, 2683 °F)
Heat of fusion 4.14  kJ·mol−1
Heat of vaporization 165  kJ·mol−1
Heat capacity (25 °C) 26.32  J·mol−1·K−1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 882 977 1097 1252 1461 1758
Atomic properties
Crystal structure hexagonal
Oxidation states 3, 1
(mildly basic oxide)
Electronegativity 1.62 (scale Pauling)
Ionization energies 1st: 589.4 kJ/mol
2nd: 1971 kJ/mol
3rd: 2878 kJ/mol
Atomic radius 190pm
Atomic radius (calc.) 156  pm
Covalent radius 148  pm
Van der Waals radius 196 pm
Magnetic ordering ???
Electrical resistivity (20 °C) 0.18 µ Ω·m
Thermal conductivity (300 K) 46.1  W·m−1·K−1
Thermal expansion (25 °C) 29.9  µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 818 m/s
Young's modulus 8  GPa
Shear modulus 2.8  GPa
Bulk modulus 43  GPa
Poisson ratio 0.45
Mohs hardness 1.2
Brinell hardness 26.4  MPa
CAS registry number 7440-28-0
Selected isotopes
iso NA half-life DM DE (MeV) DP
203Tl 29.524% Tl is stable with 122 neutrons
204Tl syn 119 Ms
(3.78 y)
β- 0.764 204Pb
ε 0.347 204Hg
205Tl 70.476% Tl is stable with 124 neutrons

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


1 gram of Thallium

Thallium (pronounced /ˈθæliəm/) is a chemical element with the symbol Tl and atomic number 81.[1] This soft gray malleable poor metal resembles tin but discolors when exposed to air. Thallium is highly toxic and is used in rat poisons and insecticides but since it might also cause cancer (although the United States EPA does not class it as carcinogen), this use has been cut back or eliminated in many countries. It is also used in infrared detectors.[2] It has even been used in some murders, earning the nicknames "The Poisoner's Poison" and "Inheritance powder" (alongside arsenic).

Notable characteristics

This metal is very soft and malleable and can be cut with a knife.

When it is first exposed to air, thallium has a metallic luster but quickly tarnishes with a bluish-gray tinge that resembles lead (it is preserved by keeping it under oil).

A heavy layer of oxide builds up on thallium if left in air.

In the presence of water, thallium hydroxide is formed.


The odorless and tasteless thallium sulfate was widely used in the past as a rat poison and ant killer. In the United States and many other countries this use is no longer allowed due to safety concerns. Other uses:

  • used in the treatment of ringworm and other skin infections. However this use has been limited due to the narrow margin that exists between toxicity and therapeutic benefit.
  • radioactive thallium-201 (half-life of 73 hours) is used for diagnostic purposes in nuclear medicine, particularly in stress tests used for risk stratification in patients with coronary artery disease A(CAD).[3][4] This isotope of thallium can be generated using a transportable generator which is similar to the technetium cow.[5] The generator contains lead-201 (half life 9.33 hours) which decays by electron capture to the thallium-201. The lead-201 can be produced in a cyclotron by the bombardment of thallium with protons or deuterons by the (p,3n) and (d,4n) reactions.[6]


Thallium has 25 isotopes which have atomic masses that range from 184 to 210. 203Tl and 205Tl are the only stable isotopes, and 204Tl is the most stable radioisotope, with a half-life of 3.78 years.

Thallium-202 (half life 12.23 days) can be made in a cyclotron[7], while thallium-204 (half life 3.78 years) is made by the neutron activation of stable thallium in a nuclear reactor.[8]


Thallium and its compounds are very toxic, and should be handled with great care. Contact with skin is dangerous, and adequate ventilation should be provided when melting this metal. Thallium(I) compounds have a high aqueous solubility and are readily absorbed through the skin. Exposure to them should not exceed 0.1 mg per m² of skin in an 8-hour time-weighted average (40-hour work week). Thallium is a suspected human carcinogen.

Part of the reason for thallium's high toxicity is that, when present in aqueous solution as the univalent thallium(I) ion (Tl+), it exhibits some similarities with essential alkali metal cations, particularly potassium (as the atomic radius is almost identical). It can thus enter the body via potassium uptake pathways. However, other aspects of thallium's chemistry are very different from that of the alkali metals (e.g., its high affinity for sulfur ligands due to the presence of empty d-orbitals), and so this substitution disrupts many cellular processes (for instance, thallium may attack sulphur-containing proteins such as cysteine residues and ferredoxins).

Thallium's toxicity has led to its use (now discontinued in many countries) as a rat and ant poison.

Among the distinctive effects of thallium poisoning are loss of hair (which led it to its initial use as a depilatory before its toxicity was properly appreciated) and damage to peripheral nerves (victims may experience a sensation of walking on hot coals). Thallium was once an effective murder weapon before its effects became understood, and an antidote (prussian blue) discovered.

Treatment and internal decontamination

One of the main methods of removing thallium (both radioactive and normal) from humans is to use Prussian blue, which is a solid ion exchange material which absorbs thallium and releases potassium. The prussian blue is fed by mouth to the person, and it passes through their digestive system and comes out in the stool.[9]


  1. thallium, Los Alamos National Laboratory. Retrieved November 21, 2006.
  2. Nayer, P. S. "Thallium selenide infrared detector". Smithsonian/NASA ADS Physics Abstract Service. Retrieved 2006-11-25. Unknown parameter |coauthors= ignored (help)
  3. Thallium Test from Walter Reed Army Medical Center
  4. Thallium Stress Test from the American Heart Association
  5. Abstract
  6. Thallium-201 production from Harvard Medical School's Joint Program in Nuclear Medicine
  7. Thallium Research from Department of Energy
  8. Manual for reactor produced radioisotopes from the International Atomic Energy Agency
  9. Prussian blue fact sheet from the Centers for Disease Control and Prevention

External links

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