Astatine (At)
Astatine (symbol At) is the chemical element with atomic number 85: a halogen, solid at 0 °C, radioactive, with an atomic mass of 210 u. Discovered in 1940 (Dale Corson, Kenneth MacKenzie and Emilio Segrè).
Astatine is a halogen, a cousin of iodine, but so radioactive that no one has ever seen a sample with the naked eye. Its most stable isotope lasts only about 8 hours. It is being studied for cancer treatments that irradiate tumors from the inside.
Key facts
- Atomic number
- 85
- Symbol
- At
- French name
- Astate
- Atomic mass
- 210 u
- Molar mass
- [210] g/mol
- Family
- Halogens
- Position
- period 6, group 17, p-block
- State at 0 °C
- Solid
- Melting point
- 301.9 °C
- Boiling point
- 336.9 °C
- Electronegativity (Pauling)
- 2.2
- First ionization energy
- 899 kJ/mol
- Atomic radius
- 127 pm
- Electron configuration
- [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵
- Radioactive
- yes
- Discovery
- 1940, Dale Corson, Kenneth MacKenzie and Emilio Segrè
Origin of the name
From the Greek astatos, unstable.
What does astatine look like?
Never observed in visible amounts: a sample would vaporize from the heat of its own radioactivity. Calculations suggest it would be a metallic-looking solid.
Where is astatine found?
It appears fleetingly in the decay chains of uranium and thorium. At any given moment, Earth's crust is thought to hold less than a gram of it in total.
Natural sources
- Fleeting traces from the decay of uranium and thorium
How is astatine produced?
Made in cyclotrons by bombarding bismuth-209 with alpha particles. Only minute quantities are produced, on the order of a billionth of a gram, and used immediately.
What is astatine used for?
- Astatine-211 for targeted alpha radiotherapy (clinical trials).
- Basic research on the chemistry of heavy elements.
In everyday life
- None: it exists for only a few hours in specialized labs
Role in living things
No biological role. Like iodine, it tends to concentrate in the thyroid gland.
Hazards of astatine
Highly radioactive, but available in such tiny amounts that the risk is confined to radiochemistry labs.
Cosmic origin
- None of it survives from the formation of the Solar System: it is constantly being created by the radioactive decay of uranium and thorium.
History of astatine
After several false claims (“alabamine,” “dakin”), it was synthesized in 1940 at Berkeley by Dale Corson, Kenneth MacKenzie and Emilio Segrè. Its name comes from the Greek astatos, “unstable.” In 1943 the Austrians Berta Karlik and Traude Bernert proved it occurs in nature.
Did you know?
- Its longest half-life, that of astatine-210, is only 8.1 hours.
- In 2013, calculations predicted that solid astatine would be a metal, unlike the other halogens.
- Despite its rarity, it has appeared on every periodic table for more than 80 years.
Isotopes of astatine
Astatine has no stable isotopes and is the rarest naturally occurring element in the Earth's crust: at any given moment there is thought to be only a minuscule amount, on the order of a gram. Its longest-lived isotope survives only about 8 hours.
- astatine-209: radioactive, half-life 5.42 h
- astatine-210: radioactive, half-life 8.1 h. The longest-lived isotope of astatine.
- astatine-211: radioactive, half-life 7.21 h. Promising for targeted alpha therapy against cancer.
Related elements
Same family (halogens): Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I).
Same column (group 17): Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Tennessine (Ts).