PTL:Thallium

From ChemEd Collaborative

Thallium

Thallium is in Group IIIA and in the 6th period. It is the heaviest element in the group and only slightly more abundant in the earth's crust than indium (In), but much less so than gallium (Ga). Other elements in the group are boron (B) and aluminum (Al).

1 Discovered
2 Name
3 Characteristics
4 Found
5 Reactions
6 Uses
7 Emission Spectra

Discovered

The first indication of the existence of thallium was given by Sir William Crookes in 1861. While examining some residues from the production of sulfuric acid for the presence of selenium and possibly tellurium, he found a beautiful green line in the emission spectrum of the material. He concluded that this could only arise from a previously unknown element, and he

Name

named the element thallium, from the Greek meaning "green branch". The actual isolation of the metal, in 1862, is usually attributed to the Frenchman Lamy, who also obtained it from the residues of the manufacture of sulfuric acid.

Characteristics

It is a gray-white, soft metal with a density of 11.8 g/cm3, much higher than that of Al (2.7 g/cm3). It is also very soft and malleable and can be cut with a knife. In the solid state, the metal has a hexagonal closest packed structure.

Found

It commonly exists as Tl(I) and Tl(III). The importance of the +1 ion is typical of p-block elements where lower oxidation numbers become more important with the heavier elements.

Reactions

The metal is attacked by air, slowly forming Tl₂O and Tl₂O₃. Like aluminum, it also forms the hydroxide when treated with base.

Uses

Thallium is used in photocells such as this thallium-activated photo-multiplier tube

The element and its compounds are extremely toxic and must be handled with care. Thallium(I) sulfate, Tl₂SO₄, has been used as a rodent and ant killer; it was used in "ant buttons" for many years. However, the compound is odorless and tasteless and so was mistakenly eaten by animals and people. For this reason the compound was banned from the market, and there are criminal penalties for its sale or use.

Owing to their toxicity, the metal and its compounds have few uses. However, the electrical conductivity of thallium sulfide changes on exposure to infrared light, so the compound is used in [photocells].