An Overview about the Chemical Element Thallium

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Symbol: Tl

Atomic Number: 81

Atomic Mass: 204.3833 amu (atomic mass units)

Melting Point: 303.5 C (576.65 K, 578.3 F)

Boiling Point: 1457.0 C (1730.15 K, 2654.6 F)

Number of Protons: 81

Number of Electrons: 81

Number of Neutrons: 123

Classification: Metallic

Crystal Structure: Hexagonal

Density @ 293 K: 11.85 grams per cubic centimeter

Color: bluish

The element thallium was discovered by the English chemist Sir William Crookes in 1861. Crookes had obtained a sample of the sludge which remained after sulfuric acid production. He extracted the element selenium from the sludge then examined the remainder with a spectrograph expecting to find tellurium. Instead of seeing the characteristic lines formed by tellurium he found a green line he'd never seen before. He named the new element which gave this line thallium from the Greek "thallus" meaning green twig. He isolated the element in the following year.

A fresh sample of thallium has a silvery luster but when exposed to air it quickly develops a bluish grey color reminiscent of lead. Longer exposure to air results in the build up of a thick oxide. If water vapor is present then a black hydride is formed by the metal. Thallium is very soft and malleable. It is so soft, in fact, that it can be cut with a knife.

The element has two stable isotopes thallium-205 accounts for 70.476% of the total abundance while the remainder is made up of thallium-203. Thallium has over forty unstable isotopes recognized with half lives ranging from 0.23 milliseconds and 3.78 years.

Thallium is found in the minerals crooksite, lorandite and hutchinsonite. The element is rarely extracted from these mineral ores but is commonly obtained as a by-product from lead or zinc refining.

Because of its tendency to form oxides and hydrides in the presence of air or water the pure metal has no industrial uses. A number of compounds of thallium have been found to be useful.

* When used in conjunction with sulfur or selenium and arsenic, thallium can produce a low melting point glass. This glass becomes liquid between 125 C (398.15 K, 275 F) and 150 C (423.15 K, 302 F).

* Thallium sulfate has been used as a rat and ant poison. Its use as such has been banned in the United States since 1974.

* Infra red detection devices use a number of thallium compounds such as thallium sulfide, thallium iodide and thallium bromide.

Thallium and all of its compounds are toxic. It has also been implicated as a causative agent of certain cancers so extreme care should be taken when handling the element. At one time thallium was used to treat ringworm but given its toxicity and the availability of new more effective antifungal drugs this is no longer used.

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