astatine

polonium ← astatine → radon I ↑ At ↓ Uus 85At Periodic table
Appearance
black solid (presumed)
General properties
Name, symbol, number	astatine, At, 85
Element category	halogens
Group, period, block	17, 6, p
Standard atomic weight	(210) g·mol−1
Electron configuration	[Xe] 4f14 5d10 6s2 6p5
Electrons per shell	2, 8, 18, 32, 18, 7 (Image)
Physical properties
Phase	solid
Melting point	575 K, 302 °C, 576 °F
Boiling point	610 K, 337 °C, 639 °F
Heat of vaporization	40 kJ·mol−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k at T/K 361 392 429 475 531 607
Atomic properties
Oxidation states	±1, 3, 5, 7
Electronegativity	2.2 (Pauling scale)
Ionization energies	1st: 890±40 kJ·mol−1
Covalent radius	150 pm
Van der Waals radius	202 pm
Miscellanea
Magnetic ordering	no data
Thermal conductivity	(300 K) 1.7 W·m−1·K−1
CAS registry number	7440-68-8
Most stable isotopes
Main article: Isotopes of astatine
iso NA half-life DM DE (MeV) DP 210At syn 8.1 h ε, β+ 3.981 210Po α 5.631 206Bi 211At syn 7.2 h
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Astatine (pronounced /ˈæstətiːn/ AS-tə-teen or /ˈæstətɨn/ AS-tətin) is a radioactive chemical element with the symbol At and atomic number 85. It is the heaviest of the discovered halogens. Although astatine is produced by radioactive decay in nature, due to its short half life it is found only in minute amounts. Astatine was first produced by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè in 1940. Three years passed before traces of astatine were also found in natural minerals. Until recently most of the physical and chemical characteristics of astatine were inferred from comparison with other elements. Some astatine isotopes are used as alpha-particle emitters in science applications, and medical applications for astatine 211 have been tested. Astatine is currently the rarest naturally-occurring element, with less than an ounce estimated to be contained in the entire Earth's crust.^[1]

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;^[2] 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. The halogens 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 the exceptionally strong hydroastatic acid. Astatine is the least reactive of the halogens, being less reactive than iodine.^[3]

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, Kenneth Ross MacKenzie, and Emilio Segrè at the University of California, Berkeley by bombarding bismuth with alpha particles.^[4]

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) by Fred Allison and associates, led to the spurious name for the element as alabamine (Ab) for a few years.^[5][6][7] This discovery was later shown to be an erroneous one.

The name Dakin was proposed for this element in 1937 by the chemist Rajendralal De working in Dhaka, Bangladesh.^[8]

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.^[9]

It took three years before astatine was found as product of the natural decay processes. The short-lived element was found by the two scientists Berta Karlik and Traude Bernert.^[10][11]

Occurrence

Astatine occurs naturally in three natural radioactive decay series, but because of its short half-life is found only 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^[12]. The most long-lived of these naturally-occurring astatine isotopes is ²¹⁹At with a half-life of 56 seconds.

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-215 atoms at any time is "only a trillion".^[13]

Production

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

Reaction	Energy of alpha particle
20983Bi + 42α → 21185At + 2 10n	26 MeV[14]
20983Bi + 42α → 21085At + 3 10n	40 MeV[14]
20983Bi + 42α → 20985At + 4 10n	60 MeV[15]

Compounds

Multiple compounds of astatine have been synthesized in microscopic amounts and studied as intensively as possible before their inevitable radioactive disintegration. The reactions are normally tested with dilute solutions of astatine mixed with larger amounts of iodine. The iodine acts as a carrier, ensuring that there is sufficient material for laboratory techniques such as filtration and precipitation to work.^[14][16]

While these compounds are primarily of theoretical interest, they are being studied for potential use in nuclear medicine.^[17] 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 (HAt), which when dissolved in water, forms hydroastatic acid.

Some examples of astatic compounds are:

• Sodium astatide (NaAt)
• Magnesium astatide (MgAt₂)
• Carbon tetraastatide (CAt₄)

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.^[18]

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. Astatine-211 is an alpha emitter with a physical half-life of 7.2 h. These features have led to its use in radiation therapy.^[19] 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.^[20]

Precautions

Since astatine is extremely radioactive, it should be handled with extreme 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 reactive, sharing similar chemical characteristics with iodine.

There are toxicologic studies of astatine-211 on mice indicating that radioactive poisoning is the major effect on living organisms. ^[21]

References

External links

Wikimedia Commons has media related to: Astatine

Look up astatine in Wiktionary, the free dictionary.

• WebElements.com - Astatine
• Doc Brown's Chemistry Clinic - Group 7 The Halogens
• Chemistry in its element podcast (MP3) from the Royal Society of Chemistry's Chemistry World: Astatine