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It's Elemental

The Element Thallium

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81 Tl Thallium 204.3833

Atomic Number: 81

Atomic Weight: 204.3833

Melting Point: 577 K (304°C or 579°F)

Boiling Point: 1746 K (1473°C or 2683°F)

Density: 11.8 grams per cubic centimeter

Phase at Room Temperature: Solid

Element Classification: Metal

Period Number: 6

Group Number: 13

Group Name: none

What's in a name? From the Greek word for a green shoot or twig, thallos.

Say what? Thallium is pronounced as THAL-ee-em.

History and Uses:

Thallium was discovered spectroscopically by Sir William Crookes, an English chemist, in 1861. Crooks had obtained the sludge left over from the production of sulfuric acid (H2SO4) from a friend. After removing all of the selenium from the sludge, he inspected it with a device known as a spectroscope to look for signs of tellurium. Rather than seeing the yellow spectral lines produced by tellurium, he observed a bright green line that no one had ever seen before. He named the new element that was producing the green line thallium, after the greek word for 'green twig', thallos. He isolated samples of thallium the next year. Thallium is found in the minerals crooksite (CuThSe), lorandite (TlAsS2) and hutchinsonite ((Pb, Tl)2As5S9), but is usually obtained as a byproduct of the production of sulfuric acid or as a byproduct of refining zinc or lead.

There are no uses for metallic thallium since pure thallium quickly combines with oxygen and water vapor from the atmosphere, forming a black, powdery substance. Thallium, used in conjunction with sulfur or selenium and arsenic, forms low melting glass. Thallium sulfate (Tl2SO4), an odorless, tasteless thallium compound, was once used as a rat and ant poison, although it has been banned from household use in the United States since 1974. Thallium sulfide (Tl2S), thallium iodide (TlI) and thallium bromide (TlBr) are all compounds used in devices to detect infrared radiation.

Estimated Crustal Abundance: 8.5×10-1 milligrams per kilogram

Estimated Oceanic Abundance: 1.9×10-5 milligrams per liter

Number of Stable Isotopes: 2 (View all isotope data)

Ionization Energy: 6.108 eV

Oxidation States: +3, +1

Electron Shell Configuration:


2s2   2p6

3s2   3p6   3d10

4s2   4p6   4d10   4f14

5s2   5p6   5d10

6s2   6p1

Citation and linking information

For questions about this page, please contact Steve Gagnon.