Characteristics Occurrence Applications Compounds Isotopes Precautions Health Effects of Thallium Treatment & Internal Decontamination Famous Uses as a Poison Thallium Data Interactive Periodic Table

1s² 2s²p⁶ 3s²p⁶d¹⁰ 4s²p⁶d¹⁰f¹⁴ 5s²p⁶d¹⁰ 6s²p¹

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.

Occurrence

Although the metal is reasonably abundant in the Earth's crust (it is 10 times more abundant than silver) at a concentration estimated to be about 0.7 mg/kg, mostly in association with Potassium minerals in clays, soils, and granites, it 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 partially water-soluble and consequentially it can spread with groundwater when soils contain large amounts of the component. Thallium can also spread by adsorption on sludge. There are indications that thallium is fairly mobile within soils.

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 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.

Applications

The oderless 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 cyrstals 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 ringwork 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 (CAD).  This isotope of Thallium can be generated using a transportable generator which is similar to the Technetium Cow.  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.
• Combined with Sulfur or Selenium and Arsenic, Thallium has been used in the production of high-density glasses that have low melting point 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 indices. 
• Thallium amalgam is used in thermometers for low temperature, because it freezes at -58°C (pure Mercury freezes at -38°C).

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.

Compounds

Isotopes

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 while thallium-204 (half life 3.78 years) is made by the neutron activation of stable Thallium in a nuclear reactor.

Precautions

Thallium and its compounds are highly 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 oer 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.  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), and so this substitution disrupts many cellular processes (for instance Thallium may attack Sulphur-containing proteins such as cysteine residues and ferredoxins).

Health Effects of Thallium

The element and its compounds are toxic and should be handled carefully.

Thallium occurs in the environment naturally in small amounts. It is not applied very widely by humans, merely as rat poison and as a substance in electro-technical and chemical industries. These applications can cause human exposure to thallium substances.

The human body absorbs Thallium very effectively, especially through the skin, the breathing organs and the digestive tract.

Thallium poisoning is mainly caused by accidental uptake of rat poison, which contains large amounts of Thallium Sulphate. Consequently, stomachaches will appear and the nervous system will be damaged.  In some cases the damage is so irreversible that death will soon follow.  When a human survives Thallium poisoning often consequences of disturbances of the nervous system, such as trembling, paralyses and behavioural changes will remain. With unborn children Thallium poisoning can cause congenital disorders.

Due to accumulation of Thallium in the bodies of humans, chronic effects consist, such as tiredness, headaches, depressions, lack of appetite, leg pains, hair loss and disturbances of the sight.

Amongst 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.

Famous Uses as a Poison

• 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 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.

• In 1953, Australian Caroline Grills was sentenced to life in prison after three family members and a close family friend died.  Authorities found Thallium in tea that she had given to two additional family members.

• Felix-Roland Mounie, a leader of the Cameroonian anticolonial armed struggle against France was murdered by Thallium poisoning on October 15, 1960.  A French agent posing as a journalist was the main suspect of this murder.

• It is claimed that South African agents once plotted to use it against Nelson Mandela while he was in prison on Robben Island.  The Truth and Reconciliation Commission heard how agents established plans to add doses of the chemical to his medication.

• In the 1960s and 1970s, Graham Frederick Young killed at least three people with Thallium. The 1995 film The Young Poisone's Handbook is based upon him.

• In 1991, Robert Curley was poisoned with Thallium.  His wife, Joann Curley, murdered him for insurance money.

• In 1995, Zhu Ling, a student at Tsinghua University in Bejing, China, was reportedly poisoned twice by her roommate, over a period of a few months.  The classmates of the victim asked for help through Usenet, to which access was very new in mainland China at the time.  Joint efforts by physicians who responded through the web led to the diagnosis of Thallium poisoning.  The case was covered by news reports around the world.  However, efforts came too late to prevent major damage: she now cannot speak, remains largely paralyzed and blind and with severely reduced mental function.

• In June 2004, 25 Russian soldiers earned Honorable Mention Darwin Awards 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.

• In 2005, a 17 year old girl in Numazu, Shizuoka, Japan, admitted to attempting to murder her mother by lacing her tea with Thallium, causing a national scandal.

(Gr. thallos, a green shoot or twig) Thallium was discovered spectroscopically in 1861 by Crookes. the element was named after the beautiful green spectral line, which identified the element.  The metal was isolated both by Crookes and Lamy in 1862 about the same time. Thallium occurs in crooksite, lorandite, and hutchinsonite.  It is also present in pyrites and is recovered from the roasting of this ore in connection iwth the production of sulfuric acid.  It is also obtained from the smelting of lead and zinc ores.  Extraction is somewhat complex and depends on the source of the thallium.  Manganese nodules, found on the ocean floor, contain thallium.  When freshly exposed to air, thallium exhibits a metallic luster, but soon develops a bluish-gray tinge, resembling lead in appearance.  A heavy oxide builds up on thallium if left in air, and in the presence of water the hydride is formed.   The metal is very soft and malleable.  It can be cut with a knife.  Thirty-two   isotopic forms of thallium, with atomic masses ranging from 176 to 210 are recognized.  Natural thallium is a mixture of two isotopes.  The element and its compounds are toxic and should be handled carefully.  Contact of the metal with skin is dangerous, and when melting the metal adequate ventilation should be provided.   Exposure to thallium (soluble compounds) - skin, as Tl, should not exceed 0.1 mg/m³ (8-hour time-weighted average - 40-hour work week).  Thallium is suspectied of carcinogenic potential for man. Thallium sulfate has been widely employed as a rodenticide and ant killer.  It is odorless and tasteless, giving no warning of its presence.   Its use, however, has been prohibited in the U.S. since 1975 as a household insecticide and rodenticide.  The electrical conductivity of thallium sulfide changes with exposure to infrared light, and this compound is used in photocells. Thallium bromide-iodide crystals have been used as infrared optical meterials.  Thallium has been used, with sulfur or slenium and arsenic, to produce low melting glasses with become fluid between 125 and 150^oC.  These glasses have properties at room temperatures similar to ordinary glasses and are said to be durable and insoluble in water.  Thallium oxide has been used to produce glasses with a high index of refraction.  Thallium has been used in treating ringworm and other skin infections; however, its use has been limited because of the narrow margin between toxicity and therapeutic benefits.  A mercury-thallium alloy, which forms a eutectic at 8.5% thallium, is reported to freeze at -60^oC, some 20 degrees below the freezing point of mercury. Commercial thallium metal (99%) costs about $40/lb.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond

Thallium was discovered spectroscopically by Sir William Crookes, an English chemist, in 1861. Crooks had obtained the sludge left over from the production of sulfuric acid (H₂SO₄) from a friend.  After removing all of the Selenium from the sludge, he inspected it with a device known as a spectroscope to look for signs of Tellurium.  Rather than seeing the yellow spectral lines produced by Tellurium, he observed a bright green line that no one had ever seen before.  He named the new element that was producing the green line Thallium, after the greek word for 'green twig', thallos.  He isolated samples of Thallium the next year.  Thallium is found in the minerals Crooksite (CuThSe), Lorandite (TlAsS₂) and Hutchinsonite ((Pb, Tl)₂As₅S₉), but is usually obtained as a byproduct of the production of Sulfuric Acid or as a byproduct of refining Zinc or Lead.

There are no uses for metallic Thallium since pure Thallium quickly combines with Oxygen and water vapor from the atmosphere, forming a black, powdery substance.   Thallium, used in conjunction with Sulfur or Selenium and Arsenic, forms low melting glass.  Thallium Sulfate (Tl₂SO₄), an odorless, tasteless Thallium compound, was once used as a rat and ant poison, although it has been banned from household use in the United States since 1974.  Thallium Sulfide (Tl₂S), Thallium Iodide (TlI) and Thallium Bromide (TlBr) are all compounds used in devices to detect infrared radiation.