Astatine (revised)

views updated May 14 2018

ASTATINE (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Astatine is a member of the halogen family, elements in Group 17 (VIIA) of the periodic table. It is one of the rarest elements in the universe. Scientists believe that no more than 25 grams exist on the Earth's surface. All isotopes of astatine are radioactive and decay into other elements. For this reason, the element's properties are difficult to study. What is known is that it has properties similar to those of the other halogens—fluorine, chlorine, bromine, and iodine. Because it is so rare, it has essentially no uses.

Discovery and naming

The periodic table is a chart that shows how the chemical elements are related to each other. The periodic table was first constructed by Russian chemist Dmitri Mendeleev (1834-1907) in the early 1870s.

SYMBOL
At

ATOMIC NUMBER
85

ATOMIC MASS
209.9871

FAMILY
Group 17 (VIIA)
Halogen

PRONUNCIATION
AS-tuh-teen

Mendeleev's periodic table contained some empty boxes. At first, no one was sure what these empty boxes meant. By the early 1900s, however, chemists had decided that the empty boxes must be spaces for elements that had not yet been discovered. A search began for elements to fill the half dozen or so boxes that still remained in the periodic table.

Two of the most troubling empty boxes were elements 85 and 87. During the first third of the twentieth century, chemists worked very hard to find these two missing elements. Along the way, a number of incorrect answers were proposed. For example, American chemist Fred Allison (1882-1974) announced in 1931 that he had discovered elements 85 and 87. He proposed the names virginium and alabamine for these two elements. (Allison was born in Virginia and worked at the Alabama Polytechnic Institute.) Unfortunately for Allison, other chemists could not repeat his experiments successfully. They decided his results must have been incorrect.

In 1940, three chemists working at the University of California at Berkeley found evidence of element 85. Dale R. Corson, Kenneth R. Mackenzie, and Emilio Segre (1905-89) found evidence of element 85 at the end of an experiment they were conducting with a cyclotron. A cyclotron is a particle accelerator, or atom smasher. In a cyclotron, small particles, such as protons, are made to travel at high speeds. The particles collide with atoms, causing the atoms to break apart into other elements.

Segrè's team suggested the name astatine for element 85 because there are no stable isotopes for the element. (See under "Isotopes" for more information.) In Greek, the word for "unstable" is astatos.

Physical and chemical properties

The properties of astatine are not well known. The element breaks down too fast to allow experiments that take more than a few hours. No melting point, boiling point, or density is specified for astatine. The most complete experiments on astatine were begun in 1966 at the Argonne National Laboratory, out-side Chicago, Illinois. Those experiments show that astatine is chemically similar to the other halogens above it in Group 17 of the periodic table. As chemists would expect, it acts more like a metal than iodine, the element just above it in the table.

Occurrence in nature

Astatine appears in the Earth's crust when the radioactive elements uranium and thorium decay. It can be made artificially only with great difficulty. By one estimate, no more than a millionth of a gram of astatine has ever been produced in the lab.

Isotopes

ALL twenty of astatine's isotopes are radioactive. That means they break down spontaneously and are transformed into other elements. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

The isotopes with the longest half life are astatine-209, astatine-210, and astatine-211. The numbers after the names here are the atomic weights of the isotopes. These isotopes have half lives of 5.5 to 8.3 hours. The half life of a radioactive isotope is the time it takes for half of a sample of the element to break down. In 5.5 hours, only half of a sample of astatine-209 will still be astatine-209. Another 5.5 hours later, only 25 percent of it will remain.

Extraction

Astatine does not occur naturally.

Uses

Astatine is far too rare to have any uses. Some research suggests a possible medical use, however. Astatine is similar to the elements above it in Group 17 (VIIA) of the periodic table, especially iodine. One property of iodine is that it tends to collect in the thyroid gland. The thyroid is a gland at the base of the neck that controls many body functions.

Some researchers think that astatine will behave like iodine. If so, it could be used to treat certain diseases of the thyroid, such as thyroid cancer. When swallowed, the astatine would go to the thyroid. There, the radiation it gives off would kill cancer cells in the gland.

No more than a millionth of a gram of astatine has ever been produced in the lab.

Compounds

There are no known commercial uses for astatine compounds.

Some researchers think that astatine could be used to treat such diseases as thyroid cancer.

Health effects

As a radioactive element, astatine would pose a serious health hazard. However, because it can be produced only artificially—and with great difficulty at that—hardly anyone would ever be exposed to it.

Chemical Elements: From Carbon to Krypton