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Category: post-transition metal Atomic number: 81 Colour: silvery white Melting point: 304°C (579°F) Boiling point: 1,473°C (2,683°F) First identified: 1861 |
Thallium is one of the most toxic elements and has been used in many murders over the years. It was the poison most often resorted to by Saddam Hussein to murder his opponents, and the ‘thallium craze’ in the early 1950s in Australia saw at least five separate crimes involving it. Until the 1970s, this deadly substance could easily be bought in the form of thallium sulphate, an insecticide and rat poison. It was discovered by William Crookes in 1861, who saw a thin green line in the spectrum of adulterated sulphuric acid, realized it must be a new element, and named it after the Greek word for a green shoot or twig: thallos.
In 1862, the French scientist Claude-Auguste Lamy did some more detailed research and purified a small amount of the soft, silver metal (which tarnishes rapidly in the air); there followed a furious controversy between Crookes and Lamy over who should get the credit for this, only settled when each was awarded their own medal.
Thallium is mainly found in ores such as potassium minerals and pollucite (with caesium). It is the similarity to potassium that makes it so dangerous. It can hijack the parts of cells that require potassium, and this interferes with the important roles played by the latter.
In the short term, thallium poisoning causes nausea and diarrhoea; over longer periods, it causes extensive nerve damage, hair loss, mental disturbances and heart failure. Strangely, the most effective antidote is potassium ‘ferrihexacyanoferrate’ (known as Prussian blue or Berlin blue), a substance that contains cyanide, but in a non-toxic form. It works by surrounding the thallium molecules and preventing them from being absorbed in the place of potassium.
Some thallium is produced as a by-product of copper and lead refining – it has few uses, other than in the electronics industry, as part of photoelectric cells, and in the form of thallium oxide to create glass with a low melting point.