(Gr. thallos, a green shoot or twig) Thallium was discovered spectroscopically in 1861
by Crookes. The element was named after the beautiful green spectral line, which
identified the element. The metal was isolated both by Crookes and Lamy in 1862 about the
Thallium occurs in crooksite, lorandite, and hutchinsonite. It is also present in
pyrites and is recovered from the roasting of this ore in connection with the production
of sulfuric acid. It is also obtained from the smelting of lead and zinc ores. Extraction
is somewhat complex and depends on the source of the thallium. Manganese nodules, found on
the ocean floor, contain thallium.
When freshly exposed to air, thallium exhibits a metallic luster, but soon develops a
bluish-gray tinge, resembling lead in appearance. A heavy oxide builds up on thallium if
left in air, and in the presence of water the hydride is formed. The metal is very soft
and malleable. It can be cut with a knife. Twenty five isotopic forms of thallium, with
atomic masses ranging from 184 to 210 are recognized. Natural thallium is a mixture of two
isotopes. A mercury-thallium alloy, which forms a eutectic at 8.5% thallium, is reported
to freeze at -60C, some 20 degrees below the freezing point of mercury.
Commercial thallium metal (99%) costs about $40/lb.
The element and its compounds are toxic and should be handled carefully. Contact of the
metal with skin is dangerous, and when melting the metal adequate ventilation should be
provided. Exposure to thallium (soluble compounds) - skin, as Tl, should not exceed 0.1
time-weighted average - 40-hour work week). Thallium is suspected of carcinogenic
potential for man.
Thallium sulfate has been widely employed as a rodenticide and ant killer. It is
odorless and tasteless, giving no warning of its presence. Its use, however, has been
prohibited in the U.S. since 1975 as a household insecticide and rodenticide. The
electrical conductivity of thallium sulfide changes with exposure to infrared light, and
this compound is used in photocells. Thallium bromide-iodide crystals have been used as
infrared optical materials. Thallium has been used, with sulfur or selenium and arsenic,
to produce low melting glasses with become fluid between 125 and 150C. These glasses have
properties at room temperatures similar to ordinary glasses and are said to be durable and
insoluble in water. Thallium oxide has been used to produce glasses with a high index of
refraction. Thallium has been used in treating ringworm and other skin infections;
however, its use has been limited because of the narrow margin between toxicity and