Astatine is a highly radioactive element and the heaviest member of halogen family. It was discovered in 1940.
History and Discovery
The existence of astatine was predicted in the halogen family under iodine by Dmitir Mendellev (1869) and by Neild Bohr. However, due to its unstable nature, it was discovered after a long tikme. Astatine was isolated as an element by Dale R. Corson, Ross MacKenzie and Emilio Serge in 1940. It was named astatos that is the Greek word for unstable [1].
Astatine
| Periodic Table Classification | Group 17 Period 6 |
|---|---|
| State at 20C | Solid |
| Color | Unknown, probably metallic |
| Electron Configuration | [Xe] 4f14 5d10 6s2 6p5 |
| Electron Number | 85 |
| Proton Number | 85 |
| Electron Shell | 2, 8, 18, 32, 18, 7 |
| Density | 0 g.cm-3 at 20°C |
| Atomic number | 85 |
| Atomic Mass | 210.00 g.mol -1 |
| Electronegativity according to Pauling | 2.20 |
Occurrence
Astatine is one of the rarest elements in the Earth’s crust [2]. It is produced as a result of decay of other elements. It is never obtained in its pure elemental form as it is so instable that it is vaporized due to its radioactivity. Due to its highly unstable nature, about less than one gram of astatine is present in the Earth’s crust in any given time. For medical purposes, astatine-211 is produced by the alpha particle bombardment of bismuth-209. As none of the isotopes, astatine-210 or astatine-211 occur naturally in a sufficient amount.
Physical Characteristics
The physical and chemical properties of astatine are not well researched. Astatine is a shiny dark colored element. Astatine is a radioactive element. It is a semiconductor and can have metallic properties. Astatine undergoes sublimation but less readily than iodine and have a low vapor pressure. Its boiling point and melting point also follow the trend of this group and are higher than other members of the halogen family.
Chemical Characteristics
Astatine is the least reactive element among its group, halogens. Chemically, astatine shows metallic properties and forms stable monatomic cations in the presence of water. various compounds of astatine are like compounds of iodine. There are only few compounds of astatine. It reacts with hydrogen to form hydrogen astatide. It also forms compounds with lead, sodium, silver and palladium [3]. It has various oxidation states, ranging from -1 to +7.
Significance and Uses
- The only commercially important isotope of astatine is astatine-211.
- It is used in the diagnosis and treatment of various cancers. It is widely studied and used in nuclear medicine.
Health Hazards
Astatine is not present in the environment that is why it poses no danger to human health. However, it is present in research laboratories and nuclear facilities, where its handling requires special precautions due to its highly radioactive nature. its exposure can cause astatine bioaccumulation in the thyroid gland. And its toxicity is like iodine.
Isotopes of Astatine
There are four naturally occurring isotopes of astatine, astatine-215, astatine-217, astatine-218 and astatine-219 [4]. Astatine-216 is the most stable natural isotope and has a half-life of 56 seconds. Astatine isotopes are continuously vaporized and are produced by the radioactive decay of uranium and thorium ores. Isotopes of astatine are very short lived, as the most stable radioactive isotope, astatine-210, has a half-life of around 8 hours. Most of the isotopes of astatine have a half-life of one second or even less. Astatine-217 is produced by the radioactive decay of neptunium-237.
REFERENCES
[1]. Corson, MacKenzie & Segrè 1940.
[2]. Emsley states that this title has been lost to berkelium, “a few atoms of which can be produced in very-highly concentrated uranium-bearing deposits”; however, his assertion is not corroborated by any primary source
[3]. Emsley, J. (2011). Nature’s Building Blocks: An A-Z Guide to the Elements (New ed.). Oxford University Press. pp. 57–58. ISBN 978-0-19-960563-7.
[4]. Lavrukhina & Pozdnyakov 1970, p. 228–229