Astatine (85)

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.

Table: basic information about and classifications of astatine.
•Name: Astatine
•Symbol: At
•Atomic number: 85
•Atomic weight: [ 210 ] 
•Standard state: solid at 298 K
•CAS Registry ID: 7440-68-8
•Group in periodic table: 17
•Group name: Halogen
•Period in periodic table: 6 
•Block in periodic table: p-block
•Color: metallic
•Classification: Semi-metallic

Historical information
Astatine was discovered by Dale Corson, MacKenzie, Segre at 1940 in California, USA. The name comes from the Greek word "astatos" meaning "unstable". Astatine was synthesized in 1940 by Dale Corson and others at the University of California, USA, by bombarding bismuth (209Bi) with α-particles. 

Physical properties 
•Melting point: 575 [or 302 °C (576 °F)] K
•Boiling point: 503 [or 230 °C (446 °F)] K
•Density of solid: 6400 (estimated) kg m-3

Orbital properties
•Ground state electron configuration: [Xe].4f14.5d10.6s2.6p5
•Shell structure: 2.8.18.32.18.7
•Term symbol: 2P3/2

Isolation
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 α-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.