What Is Astatine?
Astatine is a chemical element with the symbol At and atomic number 85, one of the five stable halogens in Group 17.
The rarest naturally occurring element. Only about 25 grams exist in Earth's entire crust.
As a halogen in Period 6 and Group 17 (Halogens), astatine's placement on the table reflects its electron configuration of [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵, which governs its bonding behavior and the type of compounds it typically forms.
Position on the Periodic Table
Astatine is located in Period 6, an unusually long row that includes the entire lanthanide series.
Within that row, astatine sits in Group 17 (Halogens), alongside elements that share a similar number of valence electrons and, by extension, similar chemical behavior.
This position is not arbitrary — the periodic table was deliberately organized so that an element's row and column reveal its expected reactivity, bonding pattern, and physical state at a glance.
Physical & Chemical Properties
Astatine has an atomic mass of (210) u and a density of ~7 g/cm³. Its melting point is 302°C and its boiling point is 337°C.
Astatine is highly reactive, readily gaining a single electron to complete its outer shell and form a stable halide ion.
Electron Configuration Explained
Astatine's electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵, describing how its electrons are distributed across shells and subshells around the nucleus.
Reading the configuration in order shows electrons filling shells of increasing energy, following the standard Aufbau principle that explains how atomic structure builds up across the periodic table.
The outermost (valence) electrons shown in this configuration are what ultimately determine astatine's chemical reactivity and the type of bonds it forms with other elements.
History & Discovery
Astatine was formally discovered in 1940, Corson, identified through the careful isolation and analytical techniques typical of that era's chemical research.
Many elements from this period were anticipated by Dmitri Mendeleev's periodic table, which successfully predicted gaps corresponding to undiscovered elements well before laboratory instruments existed to confirm them directly.
What Is Astatine Used For?
Astatine's specific properties make it useful in several applications, including:
- Cancer treatment (At-211)
- Nuclear medicine
- Research
- Targeted alpha therapy
- Radiopharmaceuticals
These uses reflect astatine's underlying classification as a halogen, since the same properties that define its category in chemistry also determine where it becomes practically useful.
Fun Facts About Astatine
- Astatine is so rare that the total amount in Earth's crust at any time is only about 25 grams.
- Astatine is element number 85 on the periodic table, in Period 6 and Group 17.
- Its standard atomic mass is (210) u, calculated from the natural abundance of its isotopes.
Frequently Asked Questions
What is Astatine's atomic number?
Astatine has atomic number 85.
What is Astatine's chemical symbol?
Astatine's symbol is At.
What group and period is Astatine in?
Astatine is in Period 6, Group 17 (Halogens).
What is Astatine's atomic mass?
Astatine's standard atomic mass is (210) u.