![Astatine element card: symbol At, atomic number 85, atomic mass [210], electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵, oxidation states +1 -1, Halogen, group 17, period 6, solid at room temperature.](https://stage.chemistry-dictionary.com/wp-content/uploads/2026/09/astatine-chemical-element.webp)
A heavier element breaks down and temporarily forms astatine before rapidly decaying even further. Astatine belongs to group 17 of the periodic table, located vertically directly below iodine. It currently holds the title as the rarest naturally occurring chemical element on Earth.
Scientists estimate that less than one single gram of astatine exists in the entire crust. This extreme scarcity is due to its very short half-life and incredibly rapid radioactive decay. Half-life is simply the time required for half of a radioactive sample to break down.
Astatine behaves chemically much like other typical halogens found in its designated periodic group. However, it exhibits noticeably more metallic properties than lighter halogens like stable solid iodine. It can readily form strong ionic bonds with various other nonmetal chemical elements during reactions.
All of its discovered isotopes are highly unstable and they decay very quickly into lighter elements. An isotope is a version of an element with a different number of atomic neutrons. Astatine-210 has the longest known half-life of all isotopes, lasting for a mere 8.1 hours.
Because of this instability, macroscopic or visible quantities have never been successfully assembled by humans. As a result, its physical properties are entirely deduced from advanced theoretical computer models. Scientists also predict its appearance and melting point using the periodic trends of lighter halogens.