Astatine (At)
Element 85 · Halogen · atomic mass [210] u · group 17, period 6
Electron shells: 2, 8, 18, 32, 18, 7
Astatine is the rarest naturally occurring element — the entire Earth's crust holds less than a teaspoon at any moment. No one has ever seen a visible quantity of it.
Any sample large enough to see would vaporise itself instantly with its own radioactive heat.
- Atomic number
- 85
- Atomic mass
- [210] u
- Electron configuration
- [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁵
- Group · period · block
- 17 · 6 · p
- Oxidation states
- −1, +1, +3, +5, +7
- Electronegativity
- 2.2 (Pauling)
- Melting point
- 302 °C
- Boiling point
- —
- Density
- 7 g/cm³
- State at 20 °C
- Solid
- Appearance
- Unknown; presumed dark solid
- Discovered
- Dale R. Corson, Kenneth Ross MacKenzie and Emilio Segrè, 1940
85 protons, and 85 electrons when neutral
Mass number of the longest-lived isotope — no standard atomic weight exists
Halogens
What Astatine is used for
- Targeted alpha therapy research for cancer
Atomic structure of Astatine
A neutral astatine atom has 85 protons and 85 electrons. Its most common isotope carries about 125 neutrons, which is where the mass number 210 comes from. Change the neutron count and you have a different isotope of astatine; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 32, 18, 7. In orbital notation that is [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁵, leaving 7 valence electrons in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Astatine's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Astatine sits in the periodic table
Group 17, one electron short of a full shell, which makes them aggressive oxidisers. They form salts directly with metals — "halogen" literally means salt-former.
- Directly above: Iodine — same group, so similar chemistry with a smaller atom.
- Directly below: Tennessine — same group, one shell larger.
- One proton lighter: Polonium.
- One proton heavier: Radon.
Discovery and naming
Astatine was discovered by Dale R. Corson, Kenneth Ross MacKenzie and Emilio Segrè in 1940. Its name comes from Greek astatos, "unstable", which is also why its symbol (At) does not match its English name.
Every isotope of astatine is radioactive, so it has no standard atomic weight. Periodic tables show the mass number of its longest-lived isotope in square brackets instead.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Astatine?
Astatine has no stable isotope, so it has no standard atomic weight. Periodic tables instead show 210 in square brackets — the mass number of its longest-lived isotope.
How many protons, neutrons and electrons does Astatine have?
Astatine has 85 protons, because the atomic number is the proton count and that is what makes an atom Astatine at all. A neutral Astatine atom has the same number of electrons, 85. The neutron count varies by isotope; the most common one has about 125, since 210 − 85 = 125.
What is the electron configuration of Astatine?
The electron configuration of Astatine is [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁵. Its electrons are arranged 2, 8, 18, 32, 18, 7 across 6 shells.
Is Astatine a metal?
No. Astatine is classified as a halogen, on the right-hand side of the table where elements gain or share electrons rather than losing them.
What group and period is Astatine in?
Astatine is in group 17 (the halogens), period 6, in the p-block. The group is its column and the period is its row.
What is the charge of Astatine?
Astatine most commonly forms a 1− or 1+ ion. It is 1 electron short of a full outer shell, so it gains one rather than losing 7. Across all its compounds it shows oxidation states of −1, +1, +3, +5, +7.
What is the melting point of Astatine?
Astatine melts at 302 °C. That puts it in the solids at everyday temperatures.
What is Astatine used for?
Targeted alpha therapy research for cancer. Astatine is the rarest naturally occurring element — the entire Earth's crust holds less than a teaspoon at any moment.
Who discovered Astatine, and where does its name come from?
Astatine (At) was discovered by Dale R. Corson, Kenneth Ross MacKenzie and Emilio Segrè in 1940. The name comes from Greek astatos, "unstable".
Is Astatine radioactive?
Yes. Every isotope of Astatine is radioactive — it has no stable form at all, which is why the periodic table shows its mass in square brackets rather than as a standard atomic weight.