Thallium

In 1861, British chemist William Crookes (inventor of the Crookes tube) was busy examining the leftovers from producing sulfuric acid. He used flame spectroscopy, a technique that had just been approved for identifying minerals after improvements by scientists like Robert Bunsen (yes, the guy behind the Bunsen burner!). Crookes was hoping to find tellurium, but instead, he spotted something totally new—a bright green line in the flame. This had never been seen before and meant he had a new element on his hands. He named it thallium, after the Greek word thallos, meaning "green twig" because of that distinctive green color.

But Crookes wasn't the only one on the trail of thallium! Around the same time, Claude-Auguste Lamy in France was also using flame spectroscopy to study materials from sulfuric acid production. Lamy noticed the same green spectral line and realized he, too, had found a new element. While Crookes was first to announce the discovery, Lamy was the first to purify thallium, producing a small nugget of the shiny new metal in 1862.

There was a bit of drama between Crookes and Lamy about who should get the most credit for discovering thallium. While Crookes made the initial find, Lamy was the first to isolate it in larger quantities, which helped scientists learn more about the element’s properties. Ultimately, both scientists were recognized for their work.

Thallium is a soft, heavy, silvery-gray post-transition metal. It can be cut with a knife at room temperature. It tarnishes very quickly when exposed to air, forming a layer of blue-gray thallium oxide. It is the heaviest of the boron group elements, and is found in group 13, period 6 of the periodic table. It can be identified by the brilliant green flame it produces in a Bunsen burner.

Thallium sulfate was commonly used as a rat poison, until people realized just how toxic it was to humans and other animals. In 1972, it was banned in the United States (after 50 years of use), when it was discovered that about one quarter of sick/dead bald eagles had been poisoned by thallium. Household use is now banned in most countries.

Thallium oxide is used to make a special glass with a high refractive index that is used in optical measuring devices. Thallium is also used in low temperature thermometers and switches, because when alloyed with mercury, it lowers its melting point by 20°C.

Thallium-201 is proving to be an excellent isotope for cardiovascular imaging, because it has a half-life of 73 hours and emits x-rays as it decays. That short half-life means that it leaves the body quickly and is less risky for patients.

Thallium compounds are also used to produce green fireworks and color artificial gemstones.

Several minerals containing thallium occur in nature as sulfide/selenide complexes with antimony, arsenic, copper, lead, and silver.

Thallium oxide, bromide, iodide, and sulfate are produced for industrial uses. Thallium usually takes a +3 or +1 oxidation state in these compounds, reflecting the number of electrons it has lost.

There are currently 41 known isotopes of thallium, with atomic masses ranging from 176 – 216. Only thallium-203 and thallium-205 are stable, and the only isotopes found naturally. Thallium-203 makes up 29.5% of naturally occurring thallium, with thallium-205 accounting for 70.5%. The most stable radioisotope of thallium is thallium-204, which has a half-life of 3.78 years.

Thallium is not found in its free form in nature, but thallium compounds do exist. The main naturally occurring ones are crookesite, hutchinsonite, and lorandite. Thallium is also found in manganese nodules on the ocean floor. Thallium is estimated to exist in the earth’s crust at an abundance of 0.7 ppm. It is mostly obtained as a by-product of sulfuric acid production from pyrites, and from refining of copper, lead, and zinc from their ores.