Astatine, (Chemical Element, Symbol At, Atomic Number 85)

Astatine is an element in Group 7 of the Periodic Table. Members Group 7 are also known as Halogens.
(The other halogens - that is, members of the same group are Fluorine, Chlorine, Bromine and Iodine. The most important difference between these elements and astatine is that astatine is highly radioactive.)

Each astatine atom consists of 85 protons, 85 electrons plus a quantity of neutrons whose exact number* depends on the particular isotope of astatine. The longest-lived isotope is Astatine-210, which has a half-life of 8.1 hours. The number of neutrons in each atom of Astatine-210 is 210 - 85 = 125.

Astatine is believed to be the rarest naturally occurring element, with less than 30 grams (1.058 ounces) estimated to be present in the entire Earth's crust.

History of Astatine / Discovery of Astatine

In the years since the the periodic table of elements became established several scientists tried to find the element following iodine in the halogen group (Group 7). The existance of this element was even predicted by Dmitri Mendeleev (who is widely accepted as the creator of the Periodic Table). Before its discovery this unknown element was called Eka-iodine because the name of the element was to be suggested by the discoverer. Discovery of "eka-iodine" was first claimed in 1931 by Fred Allison and his associates working at the then Alabama Polytechnic Institute (now Auburn University).
This element (of atomic number = 85) was therefore known as "alabamine (Ab)" for a few years until the "discovery" by Allison and his team was found to be erroneous.

There were also other claimed but later discredited "discoveries" of this element in 1937 and again in 1940. Astatine was first produced by the American physicist Dale R. Corson together with his colleagues Kenneth Ross MacKenzie, and Emilio Segrè who were all working at the University of California, Berkeley (USA) in 1940. It was not until 3 years later that traces of astatine were also found in natural minerals.

Elemental Astatine (under "standard conditions")

Elemental astatine exists only in tiny quantities and generally only for a very short period of time due to its short half-life. It's physical properties are listed as including melting point: 302 °C, boiling point: 337 °C, and density: approx 7 g/cm³. These indicate that astatine must exist as a solid at room temperature and pressure.

Astatine is the least reactive of the halogens and exhibits the most metallic properties of the halogen group.

Until recently most of the physical and chemical characteristics of astatine were inferred from comparison with other elements.

Compounds of Astatine (in general)

Several compounds of astatine have been synthesised - but only in tiny (microscopic) amounts - and studied in as much detail as possible before their complete radioactive disintegration.

In common with the other halogen elements, astatine is expected to form ionic bonds with metals such as sodium, but it can be displaced from such salts by lighter and more reactive halogens. Astatine can also react with hydrogen to form hydrogen astatide (HAt), which, when dissolved in water, forms hydroastatic acid. Examples of astatic compounds include:

• Sodium astatide (NaAt)
• Magnesium astatide (MgAt₂)
• Carbon tetraastatide (CAt₄)

Compounds of astatine are mainly of theoretical interest but are also studied for possible use in nuclear medicine.

Isotopes of Astatine

Astatine has 33 known isotopes, all of which are radioactive.
The isotopes of astatine have mass numbers ranging from 191 to 223. Some known isotopes of astatine include:

Uses of Astatine

Some astatine isotopes have been used as alpha-particle emitters in science.
Medical applications for astatine-211 have been tested.