Thallium
|
 |
Atomic Number: |
81 |
|
Melting Point: |
304 ºC |
| Atomic Symbol: |
Tl |
|
Boiling Point: |
1473 ºC |
| Atomic Weight: |
204.37
amu |
|
Density: |
11850
kg/m 3
|
| Atomic Radius: |
170.4 pm
|
|
Oxidation States: |
2 |
| Covalent Radius: |
148 pm |
|
Electron Configuration: |
[Xe]6s24f145d106p1 |
| van der Waals Radius: |
196 pm |
|
State of Matter: |
solid |
History
(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 by Lamy in 1862 at about the same time.
Properties
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. 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.
Sources
Although the metal is reasonably abundant in the Earth's crust at a
concentration estimated to be about 0.7 part per million, it exists mostly in
association with
potassium
minerals in clays, soils, and granites and, thus, is not generally considered to
be commercially recoverable from those forms.
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. In addition, several other
thallium minerals containing 16% to 60% thallium, occur in nature as sulfide or
selenide complexes with
antimony,
arsenic,
copper, lead, and
silver but are rare and have no commercial importance as sources of this
element.
Uses
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, and is used in the manufacture of photo cells. 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 therapeutic
benefits.
It is also used in semiconductor materials for selenium
rectifiers, in gamma radiation detection equipment.
Thallium is used as a high-density liquid used for sink-float
separation of minerals. Radioactive thallium-201 is used for
diagnostic purposes in nuclear medicine, particularly in stress
tests used for risk stratification in patients with coronary
artery disease (CAD).
In addition, research activity with thallium is ongoing to
develop high-temperature superconducting materials for such
applications as magnetic resonance imaging, storage of magnetic
energy, magnetic propulsion, and electric power generation and
transmission.
Isotopes
Thallium has 25 isotopes which have atomic masses that range
from 184 to 210. Tl–203 and Tl–205 are the only stable isotopes
and Tl–204 is the most stable radioisotope with a half-life of
3.78 years.
Hazards
The element and its compounds are toxic and should be handled carefully.
Amongst the distinctive effects of thallium poisoning are loss of hair, and
damage to peripheral nerves. 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 mg/m3
(8-hour time-weighted average - 40-hour work week). Thallium is suspected of
carcinogenic potential for man.