Element 81 in our International Year of the Periodic Table series is thallium. Thallium and its compounds are toxic, which limits their uses, though in the past it’s been used as a rat poison and in some types of thermometers.

Thallium sulfate was regularly used to poison rodents in the 1900s.
Of course, its availability as a rat poison also made it readily available to any would-be poisoners, and cases of both accidental and homicidal poisonings with thallium sulfate increased as a consequence over the decades after its introduction. Today, its use in rat poisons is banned in many countries and has been since the 1970s, though some countries still allow its use.

Thallium can be mixed with mercury in thermometers to measure temperatures down to –60 ˚C. Thermometers containing only mercury aren’t capable of measuring such low temperatures, as mercury freezes at –39 ˚C. This usage is now uncommon as mercury thermometers have been slowly phased out.

Today, the toxicity of thallium limits most of its usage. However, it is still used in the electronics industry in semiconductors and photoresistors. This accounts for 60-70% of thallium production. The remainder is accounted for by nuclear medicine applications and some types of glass-making.

Remember, you can keep track of all of the previous entries in this series on the site here, or on the Royal Society of Chemistry’s dedicated page.