Thallium is a metallic chemical element that is extremely toxic. Thallium is one of the poor metals because of its properties. However, the toxicity of thallium notwithstanding, this metal has several commercial as well as industrial applications. Despite its various uses, generally consumers do not come in direct contact with this metal. Generally, thallium is extracted from two minerals - crookesite and lorandite. It is easy to process this metal to produce its valuable isotopes. Since this metal is toxic, its ingestion may cause several problems. Prussian blue is the antidote for thallium ingestion.

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In its pure form, thallium is extremely soft and has a silvery white hue. It is so soft that it can actually be cut using a knife. When thallium comes in contact with air, it tarnishes rapidly and its color changes to grey and then black. Thallium's physical properties are similar to those of lead, which is yet another element in the group of poor metals. The chemical symbol of thallium is TI and its atomic number on the periodic table of elements is 81.

In fact, thallium cannot be considered as a rare metal as it is available over 10 times of silver in the nature. Thallium is found in various places and is dispersed widely. This element is generally found in potassium minerals like pollucite and sylvite.

This element partially dissolves in water and this is why it can contaminate the groundwater, especially when the soil has huge deposits of thallium. In addition, this element can also spread when it is absorbed by slush. In fact, there are hints that thallium is somewhat mobile inside the soil.

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British chemist and physicist Sir William Crookes is credited with discovering thallium. In 1861, he observed the characteristic spectroscopic signature and much later was successful in isolating thallium. Spectroscopically, this element gives out a typical green line, which led Crookes to name thallium after the Greek term "thallos", which translated into English, means “twig or new growth". Thallium is extracted from different minerals in which it is found by a smelting process.

In the past, people used thallium as a poison to get rid of insects and rats. This is primarily owing to the toxicity of the element. However, when humans were exposed to the element, it led to several ailments and, as a result, the use of thallium as an insecticide was discontinued. Today, this element continues to be used in semiconductors, photocells, high density glass with low melting point and infrared detectors. In addition, thallium isotopes are utilized in nuclear medicine for medical imaging in the form of a contrast agent. In the past, thallium was also made use of as a poison for its toxic property as only a small amount of it is enough to kill someone.

It is worth mentioning here that since thallium is extremely toxic, extreme caution is needed while handling it. Protective gears should be worn while melting or cutting this metal. Moreover, all care must be taken to avoid the metal coming in contact with the skin, as our body can soak up thallium. All products that contain this metal come with clear labels and the warning directions on the packages should be followed strictly. If anyone suspects that he or she has been exposed to thallium, they should immediately seek medical help.

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Till today, this metal does not have wide applications. Mostly people use this element as a rat poison or as a material in chemical and electro-technical industries. However, even these applications may result in exposing humans to this toxic element and cause several illnesses.

The human body can take up thallium very easily, particularly when this element comes in contact with the skin, digestive tract (when it is ingested) or the breathing organs (by inhaling the toxic substance). Mostly thallium poisoning is caused when people accidentally ingest rat poison containing this element. In fact, rat poison contains significant amounts of thallium sulphate.

In case of thallium ingestion, one will experience stomach aches and subsequently they may suffer damage of the nervous system. In some instances, the damage of the nervous system is irreversible and it may eventually result in death soon. Even if an individual survives thallium poisoning they may suffer disorders of the nervous system, for instance, paralyses, trembling and even behavioural changes will continue to affect the individual. In case, expecting mothers are exposed to this element or intake thallium, the new born may suffer from congenital problems.

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Accumulation of this element in human body may lead to chronic effects like headaches, tiredness, and lack of appetite, depression, hair loss, eye sight problems and leg pains. Moreover, thallium poisoning may also lead to pain in the joints and nerves. This usually occurs when thallium is ingested with food that has been sprayed with insecticides containing thallium.

When thallium freshly comes in contact with air, it shows a metallic lustre, but soon its color is tainted and has a bluish-green - almost similar in appearance to lead. When this metal is left uncovered in the air, a heavy oxide develops on it. In the presence of water, this oxide leads to the formation of the hydroxide.


Thallium sulphate is an odourless and tasteless substance and there was a time when people widely used it to kill ants and as a rat poison. However, the use of this substance has been banned in the United States since 1972 owing to safety concerns. In the following years, many other countries followed the US example and prohibited the use of thallium sulphate. Aside from being used as a rat poison and insecticide, people used various salts of thallium to treat ringworm and other skin infections. They were also given to tuberculosis patients with a view to reduce sweating during the night. However, this use too has been limited these days owing to these substances' constricted therapeutic index. At the same time, development of more advanced medicines to treat the condition too has limited their use for this purpose.

Thallium(I) iodide and thallium(I) bromide crystals are relatively harder compared to the common infrared optics and, hence, they have been utilized in the form of infrared optical materials. These two crystals have transmission at considerably longer wavelengths. The trade name of these materials is referred to as KRS-5. For long, people have used thallium(I) oxide for manufacturing glasses having higher refraction index. When thallium(I) oxide is combined with selenium or sulfur and arsenic, it is used for producing high-density glasses having low melting points that vary from 125°C and 150°C. These glasses possess room temperature properties which are same as the ordinary glasses and are very durable. Moreover, they are insoluble in water and possess exceptional refractive indices.

The electrical conductivity of thallium(I) sulfide alters when it is exposed to infrared light. Hence, this compound is useful for use in photo-resistors. Similarly, thallium selenide is used in a bolometer for infrared detection. When you dope selenium semiconductors with thallium, it helps to enhance their performance. Hence, this substance is used in minor quantities in selenium rectifiers. In addition, other applications of thallium doping include its use on sodium iodide crystals in devices used for detecting gamma radiation. In these applications sodium iodide crystals are doped with trace amounts of this element to enhance their effectiveness in the form of scintillation generators. It is worth noting that when a few electrodes are softened in oxygen analyzers, they contain thallium.

Before nuclear medicine started widely using technetium-99m in various applications, the isotope thallium-201, which is a radioactive substance having a half-life of 73 years, was mainly used in nuclear cardiography. In fact, radioactive thallium isotope is used even today in patients suffering from coronary artery disease for stress tests for risk stratification.

An alloy comprising thallium and mercury, which serves as a eutectic at 8.5% thallium, is said to freeze when the temperature drops to -60°C - about 20°C below mercury's freezing point. This alloy has uses in thermometers as well as low-temperature switches. Thallium(III) salts are used in organic synthesis in the form of triacetate or thallium trinitrate. These salts are effective for undertaking various transformations in aromatics, olefins, ketones and several others. In addition, thallium is also a part of the alloy used in anode plates that are found in magnesium seawater batteries. Thallium salts that are soluble are usually added to baths with gold plating with a view to accelerate the plating and also to decrease the size of grains inside the gold stratum.

A saturated solution comprising thallium(I) formate and thallium(I) malonate in equal proportions in water is called Clerici solution. This is a mobile liquid having no odour whose color changes from yellowish to almost colorless when the concentration of thallium salts is reduced in it. Clerici solution having a density of 4.25gm/cm3 at around 20°C is among the heaviest aqueous solutions known to mankind. In the twentieth century this solution was utilized to measure the density of various minerals using the floatation method. However, the use of this Clerici solution was discontinued later owing to high level of corrosiveness of the solution as well as the high toxicity of the solution.

Often, thallium iodide is used in the form of an additive when manufacturing metal-halide lamps, accompanied by one or two halides of additional metals. Using this iodide with other metal halides helps to optimize the temperature of the lamp as well as color rendering and, at the same time, moves the spectral output towards the green region - something that is necessary for underwater lighting.

As discussed above, thallium is also employed in the manufacture of special glass with low-melting point, as they serve as highly reflective lenses. In addition, in chemical research, thallium salts are utilized in the form of reagents. In many developing countries thallium sulphate is sold even today, as these countries use of this chemical as pesticides. However, its use has been prohibited in the Western nations. Thallium is also used in photocells as its electrical conductivity alters when it comes in contact with infrared light. Moreover, this metal is employed for separating minerals using the sink-float method. Thermometers contain thallium amalgam for measuring low temperature, as it has a freezing point of - 58°C – much lower compared to mercury (it is worth noting that the freezing point of unadulterated mercury is -38°C).


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