Thallium

iso	NA	half-life	DM	DE M eV	DP
²⁰³Tl	29.524%	Tl is stable with 122 neutrons
²⁰⁴Tl	{syn.}	3.78 y	Beta^- Epsilon	0.764 0.347	²⁰⁴Pb ²⁰⁴Hg
²⁰⁵Tl	70.476%	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.

Applications

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;

thallium sulfide's electrical conductivity changes with exposure to infrared light therefore making this compound useful in photocells.
thallium bromide-iodide crystals have been used as infrared optical materials.
thallium oxide has been used to manufacture glasses that have a high index of refraction.
used in semiconductor materials for selenium rectifiers,
in gamma radiation detection equipment,
high-density liquid used for sink-float separation of minerals,
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 is used for diagnostic purposes in nuclear medicine, particularly in stress tests used for risk stratification in patients with coronary artery disease (CAD).
combined with sulfur or selenium and arsenic, thallium has been used in the production of high-density glasses that have low melting points in the range of 125 and 150 �C. These glasses have room temperature properties that are similar to ordinary glasses and are durable, insoluble in water and have unique refractive indexes.

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.

History

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.

Occurrence

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.

Isotopes

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.

Precautions

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.