ASTATINE

1) INTRODUCTION :

Astatine is a radioactive chemical element with symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-lived; the most stable is astatine-210, with a half-life of 8.1 hours. A sample of the pure element has never been assembled, because any macroscopic specimen would be immediately vaporized by the heat of its own radioactivity.

SYMBOL: At

ELECTRONIC CONFIGURATION: [Xe] 4f14 5d10 6s2 6p5

Discovered: 1940




2) CHARECTERISTICS :

Astatine is an extremely radioactive element; all its isotopes have short half-lives of 8.1 hours or less, decaying into other astatine isotopes, bismuth, polonium or radon.

A) PHYSICAL PROPERTY :

Most of the physical properties of astatine have been estimated, using theoretically or empirically derived methods. For example, halogens get darker with increasing atomic weight – fluorine is nearly colorless, chlorine is yellow-green, bromine is red-brown, and iodine is dark gray/violet. Astatine is sometimes described as probably being a black solid .
The melting and boiling points of astatine are also expected to follow the trend seen in the halogen series, increasing with atomic number. On this basis they are estimated to be 575 and 610 K (302 and 337 °C; 575 and 638 °F), respectively. Some experimental evidence suggests astatine may have lower melting and boiling points than those implied by the halogen trend. Astatine sublimes less readily than does iodine, having a lower vapor pressure. Even so, half of a given quantity of astatine will vaporize in approximately an hour if put on a clean glass surface at room temperature. The absorption spectrum of astatine in the middle ultraviolet region has lines at 224.401 and 216.225 nm, suggestive of 6p to 7s transitions.
The structure of solid astatine is unknown. As an analogue of iodine it may have an orthorhombic crystalline structurecomposed of diatomic astatine molecules, and be a semiconductor (with a band gap of 0.7 eV).


B) CHEMICAL PROPERTY :


The chemistry of astatine is "clouded by the extremely low concentrations at which astatine experiments have been conducted, and the possibility of reactions with impurities, walls and filters, or radioactivity by-products, and other unwanted nano-scale interactions." Many of its apparent chemical properties have been observed using tracer studies on extremely dilute astatine solutions, typically less than 10−10 mol·L−1. Some properties – such as anion formation – align with other halogens. Astatine has some metallic characteristics as well, such as plating onto a cathode, coprecipitating with metal sulfides in hydrochloric acid and forming a stable monatomic cation in aqueous solution. It forms complexes with EDTA, a metal chelating agent, and is capable of acting as a metal in antibody radiolabeling; in some respects astatine in the +1 state is akin to silver in the same state. Most of the organic chemistry of astatine is, however, analogous to that of iodine.


3) ISOTOPES :


There are 39 known isotopes of astatine, with atomic masses (mass numbers) of 191–229. Theoretical modeling suggests that 37 more isotopes could exist. No stable or long-lived astatine isotope has been observed, nor is one expected to exist.


4) SYNTHESIS