Astatine is a radioactive chemical element with the symbol At and atomic number 85. It is the heaviest of the discovered halogens.

Characteristics

This highly radioactive element has been confirmed by mass spectrometers to behave chemically much like other halogens, especially iodine (it would probably accumulate in the thyroid gland like iodine), though astatine is thought to be more metallic than iodine. Researchers at the Brookhaven National Laboratory have performed experiments that have identified and measured elementary reactions that involve astatine; however, chemical research into astatine is limited by its extreme rarity, which is a consequence of its extremely short half-life. Its most stable isotope has a half-life of around 8.3 hours. The final products of the decay of astatine are isotopes of lead. Following the color trend of the halogens, the elements get darker in color with increasing molecular weight and atomic number. Thus, following the trend, astatine would be expected to be a nearly black solid, which, when heated, sublimes into a dark, purplish vapor (darker than iodine). Astatine is expected to form ionic bonds with metals such as sodium, like the other halogens, but it can be displaced from the salts by lighter, more reactive halogens. Astatine can also react with hydrogen to form hydrogen astatide, which when dissolved in water, forms hydroastatic acid. Astatine is the least reactive of the halogens, being less reactive than iodine.

History

The existence of "eka-iodine" had been predicted by Mendeleev. Astatine (after Greek αστατος astatos, meaning "unstable") was first synthesized in 1940 by Dale R. Corson, K. R. MacKenzie, and Emilio Segrè at the University of California, Berkeley by barraging bismuth with alpha particles.

As the periodic table of elements was long known, several scientists tried to find the element following iodine in the halogen group. The unknown substance was called Eka-iodine before its discovery because the name of the element was to be suggested by the discoverer. The claimed discovery in 1931 at the Alabama Polytechnic Institute (now Auburn University) led to the name for the element alabamine (Ab). .

The name Dakin was proposed for this element in 1937 by chemist Rajendralal De working in Dhaka.

The name Helvetium was chosen by the Swiss chemist Walter Minder, when he announced the discovery of element 85 in 1940, but changed his suggested name to Anglohelvetium in 1942.

Occurrence

Astatine occurs naturally in three natural radioactive decay series, but because of its short half-life is only found in minute amounts. Astatine-218 (²¹⁸At) is found in the uranium series, ²¹⁶At is in the thorium series, and ²¹⁵At as well as ²¹⁹At are in the actinium series. The most long-lived of these naturally-occurring astatine isotopes is ²¹⁰At with a half-life of 8.3 hours.

Astatine is the rarest naturally-occurring element, with the total amount in Earth's crust estimated to be less than 1 oz (28 g) at any given time. This amounts to less than one teaspoon of the element. Guinness World Records has dubbed the element the rarest on Earth, stating: "Only around 0.9 oz (25 g) of the element astatine (At) occurring naturally". Isaac Asimov, in a 1957 essay on large numbers, scientific notation, and the size of the atom, wrote that in "all of North and South America to a depth of ten miles", the number of astatine atoms at any time is "only a trillion".

Astatine is produced by bombarding bismuth with energetic alpha particles to obtain relatively long-lived ²⁰⁹At - ²¹¹At, which can then be distilled from the target by heating in the presence of air.

Compounds

Multiple compounds of astatine have been synthesized in microscopic amounts and studied as intensively as possible before their inevitable radioactive disintegration. While these compounds are primarily of theoretical interest, they are being studied for potential use in nuclear medicine. Astatine is expected to form ionic bonds with metals such as sodium, like the other halogens, but it can be displaced from the salts by lighter, more reactive halogens. Astatine can also react with hydrogen to form hydrogen astatide, which when dissolved in water, forms hydroastatic acid.

Some examples of astatic compounds are:

NaAt or sodium astatide

MgAt₂ or magnesium astatide

CAt₄ or carbon tetrastatide (tetraastatide)

Isotopes

Astatine has 33 known isotopes, all of which are radioactive; the range of their mass numbers is from 191 to 223. There exist also 23 metastable excited states. The longest-lived isotope is ²¹⁰At, which has a half-life of 8.1 hours; the shortest-lived known isotope is ²¹³At, which has a half-life of 125 nanoseconds.

Applications

The least stable isotopes of astatine have no practical applications other than scientific study due to their extremely short life, but heavier isotopes have medical uses. The isotope 211 of astatine is used for treating different types of tumors. Astatine 211 is an alpha emitter with a physical halflife of 7.2 h. These features have led to its use in radiation therapy. An investigation of the efficacy of astatine-211--tellurium colloid for the treatment of experimental malignant ascites in mice reveals that this alpha-emitting radiocolloid can be curative without causing undue toxicity to normal tissue. By comparison, beta-emitting phosphorus-32 as colloidal chromic phosphate had no antineoplastic activity. The most compelling explanation for this striking difference is the dense ionization and short range of action associated with alpha-emission. These results have important implications for the development and use of alpha-emitters as radiocolloid therapy for the treatment of human tumors.

Precautions

Since astatine is radioactive, it should be handled with care. Because of its extreme rarity, it is not likely that the general public will be exposed.

Astatine is a halogen, and standard precautions apply. It is less reactive than iodine, but they share similar characteristics.

References

External links

• WebElements.com - Astatine
• Los Alamos National Laboratory - Astatine
• Doc Brown's Chemistry Clinic - Group 7 The Halogens