Astatine

2008/9 Schools Wikipedia Selection. Related subjects: Chemical elements

85 polonium ← astatine → radon I ↑ At ↓ ( Uus) Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	astatine, At, 85
Chemical series	halogens
Group, Period, Block	17, 6, p
Appearance	black solid (presumed)
Standard atomic weight	(210)  g·mol−1
Electron configuration	[Xe] 4f14 5d10 6s2 6p5
Electrons per shell	2, 8, 18, 32, 18, 7
Physical properties
Phase	solid
Melting point	575  K (302 ° C, 576 ° F)
Boiling point	? 610  K (? 337 ° C, ? 639 ° F)
Heat of vaporization	ca. 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
Crystal structure	no data
Oxidation states	±1, 3, 5, 7
Electronegativity	2.2 (Pauling scale)
Ionization energies	1st: 890±40 kJ/mol
Miscellaneous
Magnetic ordering	no data
Thermal conductivity	(300 K) 1.7  W·m−1·K−1
CAS registry number	7440-68-8
Selected isotopes
Main article: Isotopes of astatine iso NA half-life DM DE ( MeV) DP 210At 100% 8.1 h ε, β+ 3.981 210Po α 5.631 206Bi
References

Astatine (pronounced /ˈæstətiːn/) is a radioactive chemical element with the symbol At and atomic number 85. It is the heaviest of the halogens.

Notable 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 product of the decay of astatine is an isotope of lead. Following the colour trends of the halogens, you will notice that the elements get darker in colour. Following the trends, astatine is 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. An earlier name for the element was alabamine (Ab).

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

Occurrence

Astatine occurs naturally from uranium-235 and uranium-238 decay, but because of its short half-life is only found in minute amounts.

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.

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

Because of the extremely short life span of astatine no practical applications exist other than scientific study.

Precautions

Since Astatine is radioactive, it should be handled with care. Because of it's 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.