Astatine (At), with the atomic number 85, holds the distinction of being the rarest naturally occurring element within the Earth’s crust. As the heaviest member of the halogen group, which also includes elements like chlorine and iodine, it exhibits chemical properties that are difficult to study due to its extreme scarcity. Its name, derived from the Greek word astatos, meaning “unstable,” encapsulates the core reason for its scarcity. This element exists in such minute, fleeting quantities that its physical appearance has never been directly observed in a macroscopic sample.
The Nuclear Reason for Scarcity
The profound scarcity of Astatine stems directly from the intense instability of its atomic nucleus. Astatine possesses no stable isotopes, meaning every single atom of the element is radioactive and destined to decay almost immediately. The most stable isotope, Astatine-210 (\(\text{At-210}\)), possesses a half-life of only 8.1 hours, which means half of any given sample will have transformed into other elements in less than a workday. The medically relevant isotope, Astatine-211 (\(\text{At-211}\)), is even shorter-lived, with a half-life of approximately 7.2 hours.
This extreme nuclear instability prevents Astatine from existing as a primordial element, unlike most other naturally occurring elements. Any \(\text{At}\) that may have been present when the Earth first formed has long since decayed away. The small, transient amounts of Astatine found in nature are created only as intermediate products within the long, complex decay chains of Uranium and Thorium ores. Specifically, isotopes like \(\text{At-218}\) and \(\text{At-219}\) briefly appear as a decay step in the Uranium-238 and Uranium-235 series, respectively. They are continuously created and destroyed at an equal rate, existing only in a state of radioactive equilibrium that maintains an infinitesimal background concentration.
Quantifying Terrestrial Presence
Based on detailed calculations of its production and decay rates within the Earth’s crust, the total global quantity is estimated to be incredibly small. While specific figures vary, the most commonly cited estimate is that the entire crust contains less than a single gram of Astatine at any given time. Other estimates suggest a total quantity across the entire Earth’s crust might be up to 25 to 30 grams.
This infinitesimal quantity makes Astatine vastly rarer than its closest competitor for the title of rarest naturally occurring element, Francium, which is estimated to have a total crustal presence of about 30 grams. The natural concentration of Astatine is so low that isolating it from ore would be practically impossible, requiring scientists to rely entirely on artificial production methods for research and application. A sample of Uranium ore containing one gram of Uranium-238 would only contain about five atoms of Astatine at any time.
Methods of Study and Production
All scientific study and application depend on artificial synthesis. Scientists obtain the necessary isotopes by using particle accelerators, specifically cyclotrons, to induce a nuclear reaction. The standard method involves bombarding a target composed of Bismuth-209 (\(\text{Bi-209}\)), the single stable isotope of Bismuth, with high-energy alpha particles.
This process forces the stable Bismuth nucleus to capture the alpha particle, which is essentially a helium nucleus, resulting in the desired Astatine isotope. The most common reaction for medical use is \(\text{Bi-209}(\alpha, 2n)\text{At-211}\), where the Bismuth nucleus absorbs the alpha particle and emits two neutrons, forming Astatine-211. Careful control of the alpha beam energy, typically around 28 to 29 MeV, is necessary to maximize the yield of the short-lived \(\text{At-211}\) while minimizing the creation of the more problematic, longer-lived \(\text{At-210}\). After irradiation, the microscopic quantities of Astatine must be separated from the bulk Bismuth target using specialized techniques like high-temperature dry distillation or wet chemistry.
Current Applications of Astatine
Despite its extreme rarity and difficulty in production, Astatine-211 is a leading candidate in the burgeoning field of Targeted Alpha Therapy (TAT). This medical application capitalizes on the specific decay properties of the isotope to precisely treat cancer. \(\text{At-211}\) emits high-energy alpha particles, which travel only a very short distance, typically less than 70 micrometers, or a few cell diameters, within human tissue.
This short range is highly advantageous because it allows the alpha particles to deliver a massive and localized dose of radiation, effectively destroying cancerous cells, while sparing surrounding healthy tissue from significant collateral damage. The 7.2-hour half-life of \(\text{At-211}\) is also beneficial, as it is long enough to allow for synthesis, transportation, and delivery to the tumor, but short enough that any residual radioactivity clears the body rapidly after treatment. Astatine is chemically bound to a targeting molecule, such as an antibody or peptide, which guides the radionuclide directly to the cancer cells.