|Name: astatine||Group number: 17|
|Symbol: At||Group name: Halogen|
|Atomic number: 85||Period number: 6|
|Atomic weight: [ 210 ]||Block: p-block|
|CAS Registry ID: 7440-68-8||Voice: |
|Standard state: solid at 298 K||Colour: metallic|
|Classification: Semi-metallic||Availability: |
This sample of uranite contains a vanishingly small amount of astatine. Image adapted with permission from Prof James Marshall"s (U. North Texas, USA) Walking Tour of the elements CD.
The longest-lived isotope, 210At, has a half-life of only 8.3 hours. There are about 20 isotopes known, all of which are radioactive. Astatine is a halogen and possibly accumulates in the thyroid like iodine.
IsolationHere is a brief summary of the isolation of astatine.
Astatine is radioactive and essentially unavailable in nature. It is not possible to make other than in a nuclear reactor. Bombardment of the bismuth isotope 20983Bi with a-particles (helium nuclei, 42He) results in formation of shortlived astatine and neutrons. The bismuth target is cooled during irradiation to prevent the volatile astatine disappearing.
20983Bi + 42He 21185At + 2 10n
The 211At isotope has a half life of just over 7 hours so it is necessary to work quickly with it! Available quantities are of the order of 0.001 mg.
Heating the bismuth target to 300-600°C under N2 results in a stream of the elemental astatine that can be collected on a cold glass finger.