mercury - thallium - lead
Name, Symbol, NumberThallium, Tl, 81
Chemical series Poor metals
Group, Period, Block13 (IIIA), 6, p
Density, Hardness 11.85 Mg/m³, 1.2
Appearance Silvery white
Missing image

Atomic properties
Atomic weight 204.3833 g/mol
Atomic radius (calc.) 190 (156) pm
Covalent radius 148 pm
van der Waals radius 196 pm
Electron configuration [Xe]4f14 5d10 6s2 6p1
es 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 cm³/mol
Heat of vaporization 164.1 kJ/mol
Heat of fusion 4.142 kJ/mol
Vapor pressure 5.33 µPa at 577 K
Speed of sound 818 m/s at 25 ?C
Electronegativity 1.62 (Pauling scale)
Specific heat capacity 129 J/(kg·K)
Electrical conductivity 6.17 MS/m
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 fJDP
203Tl29.524%Tl is stable with 122 neutrons
204Tl{syn.}119 MsBeta
205Tl70.476%Tl is stable with 124 neutrons
SI units & STP are used except where noted.

Thallium is the chemical element in the periodic table that has the symbol Tl and atomic number 81. This soft gray malleable poor metal resembles tin but discolors when exposed to air. Thallium 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.

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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 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;

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 mg/kg, 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 derives from its ability to replace important alkali metal cations such as sodium and potassium in the body; all these metals share a common oxidation state of +1. This substitution disrupts many cellular processes. 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. Thallium was once an effective murder weapon before its effects became understood and an antidote (prussian blue) discovered.

In June 2004, at least 25 Russian soldiers became sick after thallium exposure when they found a can of Thallium powder in a rubbish dump near their base at Khabarovsk in the Russian Far East. Unaware of the danger, the conscripts added it to the tobacco they smoked and used it as a substitute for talcum powder on their feet.

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 1995 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.

The CIA is believed (by its Inspector General) to have conceived (but not executed) a scheme to poison Fidel Castro by exposure to thallium salts (placed in his shoes while they were being polished). The goal was to discredit him by causing him to lose is characteristic hair and beard. The scheme progressed as far as testing on animals, but the trip during which the poison was to be administered fell through.


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