Astatine is a chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust. Astatine appears only as a decay product of heavier elements.
All isotopes of astatine are short-lived. The most stable isotope, astatine-210, has a half-life of only 8.1 hours. Because of this brief existence, a solid sample of astatine has never been observed, as any macroscopic amount would vaporize due to its own radioactivity.
The physical and chemical properties of astatine remain largely a mystery. Many characteristics have been estimated based on its position in the periodic table, where it is positioned as a heavier analog to the stable halogens. It may display both metallic and nonmetallic behaviors, and its potential appearance is hypothesized to be either dark or lustrous.
Astatine is classified as an extremely radioactive element. Its isotopes decay into others, including bismuth and polonium, at rates that make research challenging. The only other element that is less stable than astatine is francium, and even the more stable isotopes of astatine are usually synthetic, produced through bombardment of bismuth.
Due to its radioactive nature, any piece of astatine would vaporize itself almost immediately. Estimations suggest it could potentially be deposited as a thin film, but this has not yet been achieved.
The chemistry of astatine is obscured by its rarity. Most experiments have been conducted on extremely dilute solutions, which complicate the understanding of its properties. Some chemical traits align it with other halogens, while others reveal metallic characteristics.
Astatine's electronegativity measures 2.2, placing it below iodine. The compound hydrogen astatide has been suggested, where the negative charge resides on the hydrogen atom rather than on astatine.
The existence of astatine is fleeting and shrouded in uncertainty. It is a prime example of the bizarre extremes of the natural world.
