Main Page | See live article | Alphabetical index


Name, Symbol, NumberThallium, Tl, 81
Chemical series True metals
Group, Period, Block13 (IIIA), 6 , p
Density, Hardness 11850 kg/m3, 1.2
Appearance Silvery white
Atomic Properties
Atomic weight 204.3833 amu
Atomic radius (calc.) 190 (156) pm
Covalent radius 148 pm
van der Waals radius 196 pm
Electron configuration [Xe]44f14 5d10 6s2 6p1
e- 's per energy level2, 8, 18, 32, 18, 3
Oxidation states (Oxide) 3,1 (mildly basic)
Crystal structure Hexagonal
Physical Properties
State of matter solid
Melting point 577 K (579 F)
Boiling point 1746 K (2683 F)
Molar volume 17.22 ×1010-3 m3/mol
Heat of vaporization 164.1 kJ/mol
Heat of fusion 4.142 kJ/mol
Vapor pressure 5.33 E-06 Pa at 577 K
Speed of sound 818 m/s at 293.15 K
Electronegativity 1.62 (Pauling scale)
Specific heat capacity 129 J/(kg*K)
Electrical conductivity 6.17 106/m ohm
Thermal conductivity 46.1 W/(m*K)
1st ionization potential 589.4 kJ/mol
2nd ionization potential 1971 kJ/mol
3rd ionization potential 2878 kJ/mol
Most Stable Isotopes
isoNAhalf-life DMDE MeVDP
203Tl29.524%Tl is stable with 122 neutrons
204Tl{syn.}3.78 yBeta-
205Tl70.476%Tl is stable with 124 neutrons
SI units & STP are used except where noted.
Thallium is a chemical element in the periodic table that has the symbol Tl and atomic number 81. This soft gray malleable true metal resembles tin but discolors when exposed to air. Thalium is highly toxic and is used in rodent and insect poisons but since it can also cause cancer, this use has been cut back or eliminated in many countries. It is also used in infrared detectors.

Table of contents
1 Notable Characteristics
2 Applications
3 History
4 Occurrence
5 Isotopes
6 Precautions
7 External Links

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 water). A heavy layer of oxide builds up on thallium if left in air, and in the presence of water thallium hydride 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, however. Other uses; In addition, research activity with thallium is ongoing to develop high-temperature superconducting materials for such applications as magnetic resonance imaging, storage of magnetic energy, magnetic propulsion, and electric power generation and transmission.


Thallium (
Greek thallos meaning "a green shoot or twig") was discovered by Sir William Crookes in 1861 in England while he was making spectroscopic determinations for tellurium on residues from a sulfuric acid plant. The name comes from Thallium's bright green spectral emission lines. In 1862 Crookes and Claude-Auguste Lamy isolated the metal independent of each other.


Although the metal is reasonably abundant in the
Earth's crust at a concentration estimated to be about 0.7 part per million, it exists mostly in association with potassium minerals in clays, soils, and granites and, thus, is not generally considered to be commercially recoverable from those forms. The major source of commercial thallium is the trace amounts found in copper, lead, zinc, and other sulfide ores.

Thallium is found in the minerals crooksite, hutchinsonite, and lorandite. This metal is also contained in pyrites and is extracted as a by-product of sulfuric acid production when pyrite ore is roasted. Another way this element is obtained is from the smelting of lead and zinc rich ores. Manganese nodules which are found on the ocean floor, also contain thallium but nodule extraction is prohibitively expensive and potentially environmentally destructive. In addition, several other thallium minerals containing 16% to 60% thallium, occur in nature as sulfide or selenide complexes with antimony, arsenic, copper, lead, and silver but are rare and have no commercial importance as sources of this element.


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


Thallium and its compounds are highly toxic and should be handled with great care. The toxicity has led to its use (now discontinued in many countries) as a
rat poison. Amongst the distinctive effects of thallium poisoning are loss of hair, and damage to peripheral nerves. Contact with skin is dangerous and adequate ventilation should be provided when melting this metal. Exposure to soluble compounds of thallium shouldn't exceed 0.1 mg per m of skin in an 8-hour time-weighted average (40-hour work week). Thallium is a suspected human carcinogen.

The hair loss was the cause of thallium being used in the treatment of ringworm, but that use has long been abandoned. There is a persistent story that the CIA, in an attempt to discredit Fidel Castro had actually planned to poison him with thallium so that his beard would fall out, but apparently sanity prevailed and the attempt was never made.

The detective fiction writer, Agatha Christie, who had worked as a pharmacist, used thallium as the agent of murder in her novel The Pale Horse - the first clue to the murder method coming from the hair loss of the victims.

The 1996 film The Young Poisoner's Handbook was based on the activities of Graham Frederick Young who killed at least three people with thallium in the 1960s and 1970s.

External Links