The image reflects the origin of the element’s name (from Greek ‘thallos’, meaning ‘a green shoot or twig’), its toxicity and its use in the manufacture of reflective glass.
| Density | 11.8 |
| Melting Point | 304°C |
| Boiling Point | 1473°C |
The use of thallium is limited as it is a toxic element. Thallium sulfate was employed as a rodent killer – it is odourless and tasteless – but household use of this poison has been prohibited in most developed countries.
Most thallium is used by the electronics industry in photoelectric cells. Thallium oxide is used to produce special glass with a high index of refraction, and also low melting glass that becomes fluid at about 125K.
An alloy of mercury containing 8% thallium has a melting point 20°C lower than mercury alone. This can be used in low temperature thermometers and switches.
The discovery of thallium was controversial. William Crookes of the Royal College of Science in London was the first to observe a green line in the spectrum of some impure sulfuric acid, and realised that it meant a new element. He announced his discovery in March 1861 in Chemical News. However, he did very little research into it.
Meanwhile, in 1862, Claude-August Lamy of Lille, France, began to research thallium more thoroughly and even cast a small ingot of the metal itself. The French Academy now credited him its discovery. He sent the ingot to the London International Exhibition of 1862, where it was acclaimed as a new metal and he was awarded a medal. Crookes was furious and so the exhibition committee awarded him a medal as well.