Astatine (At) is a chemical element with atomic number 85. Out of all elements that occur naturally on Earth, astatine is the rarest. It exists as part of the decay chains of heavier elements. Astatine has a short half-life and quickly decause into other elements. Its name comes from astatos, the Greek word for unstable. Out of the Earth's total mass, which is roughly 6 septillion kilograms, it estimated that less than one gram is Astatine!

Being both very rare and very short-lived, its properties are not known with certainty, but it belongs to the group of chemical elements called halogens.

Astatine can be spelled using chemical element symbols: As - Ta - Ti - Ne.