Anders Lennartson ponders on the contribution of thallium to society, since its main characteristic is its toxicity.
The lightest member of group 13 in the periodic table, boron, has been used in medicine for centuries and serves a wide variety of industrial applications; what would a chemist do without borosilicate glass? Aluminium, the next family member, is used for everything from wrapping chocolate bars to constructing space craft. Then follow gallium and indium, two elements that are indispensable in modern electronics — for instance, InGaN is used as the emitting layer in blue light-emitting diodes. At the end of the group comes thallium, the black sheep. Its compounds have found some use, for example in photoresistors, optical glass and in a class of high-temperature super conductors, yet element 64 has no real widespread application in consumer products and it appears that we could live very well — and perhaps even be better off — without it altogether.
The story begins in 1861 when Sir William Crookes used the newly invented spectroscope to search for tellurium in a residue from a sulfuric acid plant. He did not find any, but instead noticed a green, as-yet-unknown spectral line flashing by. Crookes named the new element thallium, after the Greek θαλλός meaning green branch. Whether it was Crooks or the French chemist Claude-Auguste Lamy — also working, independently, on materials obtained from a sulfuric acid plant — who first prepared metallic thallium is disputed1. At the time this led to a major controversy across the Channel, which seems to have died down after both scientists received recognition.
In contrast to the lighter group 13 elements, thallium prefers the +1 rather than the +3 oxidation state. Due to a similar ionic radius to K+ our bodies readily take up Tl+, with an often fatal result as it disrupts important processes involving K+, although the exact mechanisms are not yet known. Ingestion of usually innocent-looking, colourless, nearly tasteless thallium salts gives rise to gastric and neurological disorders and rapid organ degradation. At low doses, symptoms are slow to appear and can easily be attributed to other ailments, which makes thallium rather discreet and why it is dubbed the poisoner's poison. A characteristic effect of thallium poisoning is massive loss of hair.
Soon after its discovery, thallium's toxicity became apparent and it found widespread use as rat poison, but has since been deemed unsafe and banned in many countries due to numerous tragic accidents as well as murders. Agatha Christie made use of thallium in her 1961 novel The Pale Horse — and in doing so actually saved a life. In 1977, a 19-month-old girl from Qatar was admitted to Hammersmith Hospital, London, suffering from a serious, unknown sickness. Without a diagnosis, the doctors could not do much. One of the nurses, however, was reading The Pale Horse and realised the similarities between the symptoms of her patient and Agatha Christie's fictitious victim. A urine sample revealed high levels of thallium and an antidote could be administrated — Prussian blue, which binds to the metal and helps expel it from the body. The subsequent publication2 describing the case includes the following acknowledgement: “We are indebted to the late Agatha Christie for her excellent and perceptive clinical descriptions and to Nurse Maitland for keeping us up to date on the literature.”
English serial killer Graham Young (1947–1990), 'the teacup poisoner', also built his fame on thallium3. Aged 14 he killed his step-mother with thallium acetate, but was found out when he started to treat his father in a similar fashion, and ended up at the Broadmoor psychiatric hospital. Released at the age of 23, he took up his old hobby and it didn't take long until a mysterious disease started to spread among his colleagues, eventually killing two. It was Young himself who suggested thallium poisoning to the company doctor, thus putting investigators on the right track. He spent the remainder of his life behind bars.
Considering thallium's non-selective toxicity and limited applications, perhaps it would have been no great loss if, rather than causing a tussle over the credit for its discovery, the green spectral line had remained unnoticed.
Gmelins Handbuch der Anorganischen Chemie. Thallium (Verlag Chemie, 1940).
Matthews, T. G. & Dubowitz V. Brit. J. Hosp. Med. 17, 607–608 (1977).
Emsley, J. The Elements of Murder. A History of Poison (Oxford Univ. Press, 2005).
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Lennartson, A. Toxic thallium. Nature Chem 7, 610 (2015). https://doi.org/10.1038/nchem.2286