ElementsWhat is astatine?
Astatine is a radioactive element and the rarest that occurs naturally. Only a tiny amount exists in the Earth's crust at any moment, and no one has seen a visible piece of it.
What is astatine used for?
No everyday uses. A radioactive form is being studied for cancer treatment.
Where the name comes from
From the Greek word astatos, unstable.
Key facts
- Symbol
- At
- Atomic number
- 85
- Atomic mass
- 209.98715 u
- Family
- Halogen
- Period
- 6
- Group
- 17
- Block
- p
- State at room temperature
- Solid
- Melting point
- 301.9 °C (575 K, 575 °F)
- Boiling point
- not measured
- Density
- 7 g/cm³
- Electrons per shell
- 2, 8, 18, 32, 18, 7
- Electron configuration
- [Xe]6s2 4f14 5d10 6p5
- Common oxidation states
- 7, 5, 3, 1, -1
- Electronegativity (Pauling)
- 2.2
- First ionisation energy
- 9.5 eV
- Electron affinity
- 2.8 eV
- Atomic radius (van der Waals)
- 202 pm
- Discovered
- 1940
Place in the periodic table
Astatine is element number 85. It sits in period 6 (row 6) of the periodic table, in group 17 (column 17), and belongs to the p-block. Just before it comes polonium (84), and just after it comes radon (86). In the same column, iodine is above it and tennessine is below it. Elements in one column tend to behave alike.
Family: Halogens.
Physical properties
At room temperature astatine is a solid. Its melting point is about 301.9 °C. Its boiling point has not been measured reliably. Its density is 7 grams per cubic centimetre, so a one-litre block would weigh about 7 kilograms. By density it ranks 50th among the 96 elements with a measured value.
Inside the atom
An atom of astatine has 85 protons in its nucleus and, when neutral, 85 electrons around it. The electrons are arranged in 6 shells: 2, 8, 18, 32, 18, 7. The outermost shell holds 7 electrons. Written in full, the electron configuration is [Xe]6s2 4f14 5d10 6p5.
Its electronegativity on the Pauling scale is 2.2. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 9.5 electronvolts (about 917 kilojoules per mole). One mole of astatine weighs 209.98715 grams, so a single gram contains about 2.87 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains astatine with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.