258 | The History and Use of Our Earth’s Chemical Elements Properties

258 | The History and Use of Our Earth’s Chemical Elements Properties

Astatine is located just below iodine, which suggests that it should have some of the same chemical properties as iodine, even though it also acts more like a metal or semimetal than does iodine. It is a fairly heavy element with an odd atomic number, which assisted chemists in learning more about this extremely rare element. The 41 isotopes are man-made in atomic reactors, and most exist for fractions of a second. The element’s melting point is about 302°C,

its boiling point is approximately 337°C, and its density is about 7g/cm 3 . Characteristics Astatine is the heaviest and densest of the elements in group 17 (VIIA). It is difficult to

determine the chemical and physical properties and characteristics of astatine because it is present in such small quantities that exist for extremely short periods of time. Many of its characteristics are inferred through experiments rather than by direct observations.

Abundance฀and฀Source Chemists of the early twentieth century tried to find the existence of element 85, which

was given the name “eka-iodine” by Mendeleev in order to fill the space for the missing ele- ment in the periodic table. Astatine is the rarest of all elements on Earth and is found in only trace amounts. Less than one ounce of natural astatine exists on the Earth at any one time. There would be no astatine on Earth if it were not for the small amounts that are replenished by the radioactive decay process of uranium ore. Astatine produced by this uranium radioac- tive decay process soon decays, so there is no long-term build up of astatine on Earth. The isotopes of astatine have very short half-lives, and less than a gram has ever been produced for laboratory study.

History Early twentieth-century chemists were able to ascertain some of the properties of astatine

from its position in the periodic table and from the fact that is was heavy and has an odd atomic number. At the beginning of World War II in 1940, Dale Raymond Corson (1914– 1995), K. R. Mackenzie (1912–1995), and Emilio Gino Segre (1905–1989) produced a new element with 85 protons by using a cyclotron. Although the war interrupted their work, it was continued and confirmed in 1945 when they created astatine in the laboratory by firing high-energy alpha particles (helium nuclei) at a target of bismuth-209. This method is still used today for the production of small portions of astatine-211 plus two neutrons.

There is some question as to who discovered astatine. Some authorities state that Fred Allison and E. J. Murphy discovered astatine in 1931, but most give Corson, Mackenzie, and Segre the credit.

Astatine filled the next-to-last gap in the periodic table; at the time, element 61, prome- thium, had not yet been discovered.

Common฀Uses In water solution astatine resembles iodine in some of its chemical and physical properties.

Both are powerful oxidizing agents. It has limited use in medicine as a radioactive source. It concentrates in the thyroid gland just like iodine, which makes it a useful radioisotope tracer.

259 Because of its isotopes’ short half-lives and its scarcity, astatine has few practical uses out-

Guide to the Elements |

side of the laboratory for research, where less than a gram has ever been produced. Examples฀of฀Compounds

Astatine does not have stable or useful compounds. Like a halogen, it will form halogen salts with a few other elements. No significant astatine commercial compounds have been produced with the exception of astatine-211, which has a half-life of just over seven hours and is used as a radioactive tracer for thyroid diseases.

Hazards The major hazard is from the radiation of astatine’s isotopes. However, given that these

isotopes have very short half-lives, they do not pose a great long-term danger. Even so, astatine is considered a dangerous element that is a radioactive poison and carcinogen. It has been demonstrated that astatine causes cancer in laboratory animals.

The Noble Gases (Inert Gas Elements): Periods 2 to 6, Group 18 (VIIIA)