Thallium

Thallium is a soft, blue-grey toxic metal that was once a popular rat poison and murder weapon (it is colourless, tasteless and causes death slowly over days). Today it is used in specialised infrared optics, heart imaging tracers and some electronic applications.

  • Atomic Number8181 protons, 81 electrons
  • Atomic Mass204.383 u81× heavier than hydrogen
  • State at Room TempSolidSolid
  • Density11.8 g/cm³
  • Melting / Boiling303.9°C / 1472.8°C
  • Discovered1861

What is Thallium?

Thallium is a post-transition metal in Group 13 below indium. With 81 protons, it behaves like lead and mercury in toxicity, accumulating in the body and disrupting potassium transport. Its name comes from the Greek thallos meaning green twig, because of the vivid green spectral line used to discover it. William Crookes identified it spectroscopically in 1861.

Discovered in 1861 by William Crookes using spectroscopy, independently confirmed by Claude-Auguste Lamy who isolated the pure metal. Named from the Greek thallos (green twig) for its brilliant green spectral line.

Fact Thallium was called "the poisoner's poison" because thallium sulfate is colourless, odourless and tasteless, with symptoms that mimic other diseases. It was sold as rat poison in Europe until the 1970s and was used in numerous murders. The KGB reportedly used thallium to poison dissidents. A well-known case involved the poisoning of Iraqi Kurdish villagers in 1988.

Where you find Thallium

On Earth

By-product of zinc and lead smelting, and of coal burning. China, Kazakhstan and Russia are producers.

How we use Thallium

  • Thallium-201 is used as a radioactive tracer in nuclear medicine cardiac imaging to assess blood flow to the heart muscle.. Medical imaging
  • Thallium-containing glasses and crystals are transparent to infrared light and used in IR spectroscopy and thermal imaging cameras.. Infrared optics
  • Thallium alloys reduce the melting point of mercury in certain low-temperature thermometers.. Scientific instruments
Did you know? Thallium was called "the poisoner's poison" because thallium sulfate is colourless, odourless and tasteless, with symptoms that mimic other diseases. It was sold as rat poison in Europe until the 1970s and was used in numerous murders. The KGB reportedly used thallium to poison dissidents. A well-known case involved the poisoning of Iraqi Kurdish villagers in 1988.

How it was discovered

Discovered in 1861 by William Crookes using spectroscopy, independently confirmed by Claude-Auguste Lamy who isolated the pure metal. Named from the Greek thallos (green twig) for its brilliant green spectral line.

Deeper dive: thallium properties and applications

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Moving to 82 protons on the periodic table takes us to the next element.