Astatine (At), atomic number 85, is one of the most enigmatic elements on the periodic table. It is widely recognized as the rarest naturally occurring element in Earth’s crust. Its elusive nature prompts questions about its scarcity and study difficulties. The mystery surrounding astatine stems from a combination of its inherent properties and the conditions under which it forms and exists.
Astatine’s unique status is due to its extreme instability. All of astatine’s isotopes are radioactive, decaying rapidly. This continuous transformation into other elements means that any astatine present is constantly disappearing, making it exceptionally challenging to observe or collect in any significant quantity.
The Nature of Astatine’s Instability
Astatine’s rarity is directly linked to its nuclear instability, reflected in its name, from the Greek “astatos” meaning “unstable.” Every known isotope of astatine is radioactive, meaning its atomic nuclei spontaneously transform into different elements by emitting radiation. This process of radioactive decay ensures that astatine does not persist for long in any form.
Understanding this instability involves “half-life,” the time for half of a radioactive substance to decay. For astatine, half-lives are remarkably short, ranging from fractions of a second to a few hours for its most stable isotopes. The most stable isotope, astatine-210, has a half-life of approximately 8.1 hours.
This short half-life means that if you had a quantity of astatine-210, half of it would decay away in just over eight hours, and then half of the remainder would decay in another eight hours, and so on. Such rapid decay prevents astatine from accumulating in weighable amounts, as it transforms almost as quickly as it is produced. This fundamental property makes it difficult to study and limits its presence in the environment.
Its Fleeting Natural Existence
Despite its instability, astatine exists naturally in minute, fleeting quantities. It is not found in deposits, but forms through the radioactive decay of heavier elements. Specifically, astatine isotopes are continuously produced as intermediate products within the decay chains of naturally occurring uranium and thorium ores.
As uranium and thorium atoms undergo radioactive transformations, they occasionally produce astatine isotopes as temporary byproducts. These atoms exist for a brief period before decaying further into other elements, such as polonium, bismuth, or radon. This constant cycle of formation and decay means that any astatine present in nature is always in flux, never accumulating into significant amounts.
Estimates suggest that the total amount of astatine present in Earth’s crust at any given time is less than one gram, with some sources indicating less than 50 milligrams in the top kilometer. This makes it the rarest naturally occurring element, with its presence being a result of ongoing nuclear processes rather than stable geological formations. Its natural occurrence is thus a dynamic equilibrium of continuous creation and immediate disappearance.
Challenges in Production and Detection
Astatine’s rarity and instability pose challenges for scientists. Since it cannot be mined or collected, researchers synthesize it in laboratories. The primary method involves bombarding bismuth-209 with energetic alpha particles in a particle accelerator, such as a cyclotron.
This nuclear reaction produces specific isotopes of astatine, such as astatine-211, which has a half-life of about 7.2 hours. Even with powerful accelerators, only minuscule quantities, in nanograms or picograms, can be produced. For example, a total of approximately 0.05 micrograms of astatine have been produced to date.
Working with such minuscule, radioactive samples presents obstacles. The intense radioactivity means that any substantial quantity would vaporize due to its own heat, making direct observation of its physical appearance impossible. Scientists must rely on highly sensitive detection techniques and tracer studies, where infinitesimally small amounts are used, or infer properties from theoretical models and its behavior in extremely dilute solutions.