Thallium

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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 rat poisons and insecticides but since it might also cause cancer (although the 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.

1 Notable characteristics
2 Applications
3 History
4 Occurrence
5 Isotopes
6 Precautions
7 Famous uses
8 See also
9 References
10 External links

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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. In the presence of water, thallium hydroxide is formed.

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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. 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.
thallium amalgam is used in thermometers for low temperature, because it freezes at -58 °C (pure mercury freezes at -38 °C).
illicit production of methamphetamines.

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.

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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.

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Occurrence

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 crookesite (TlCu₇Se₄), hutchinsonite(TlPbAs₅S₉), and lorandite (TlAsS₂). 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.

see also category:Thallium minerals

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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.

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Precautions

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 (victims may experience a sensation of walking on hot coals). 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 m³ 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.

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Famous uses

Image:Thallium rod corroded.jpgIn June 2004, 25 Russian soldiers earned Darwin Awards or Honorable Mentions after becoming ill from thallium exposure when they found a can of mysterious white powder in a rubbish dump on their base at Khabarovsk in the Russian Far East. Oblivious to the danger of misusing an unidentified white powder from a military dump site, the conscripts added it to tobacco, and used it as a substitute for talcum powder on their feet.

The CIA is believed (by its Inspector General) to have conceived 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 his 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.

Agatha Christie, who worked as a pharmacist, used thallium as the agent of murder in her detective fiction 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 Australian serial killer Caroline Grills was known as "Auntie Thally" for her choice of Thallium as an agent to poison her victims.

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