At · Element 85 · Metalloid
The rarest naturally occurring element on Earth — perhaps 25 grams exist in the entire crust at any moment.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where astatine sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All astatine atoms have the same number of protons, 85, and differ only in their number of neutrons: those variants are its isotopes. None of them is stable. The longest-lived known isotope, At-210, has 125 neutrons and a half-life of 8.1 hr.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| At-207 | 122 | — | 1.81 hr | ε/β⁺ (91.4%), α (8.6%) |
| At-208 | 123 | — | 1.63 hr | ε/β⁺ |
| At-209 | 124 | — | 5.42 hr | ε/β⁺ (95.9%), α (4.1%) |
| At-210 longest-lived | 125 | — | 8.1 hr | ε/β⁺ |
| At-211 | 126 | — | 7.214 hr | ε (58.2%), α (41.8%) |
Here the classroom shortcut, rounded atomic mass (210) minus atomic number, gives the same answer: 125.
Decay modes
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 150 pm — that is 0.15 nm, so about 3.33 million of them side by side would span a millimetre.
Thermal range
Astatine is liquid over a 35 K window, from 575 K to 610 K.
The story
Astatine (At) is element 85, the rarest naturally occurring element in Earth's crust and the heaviest naturally occurring member of group 17, the halogen group, below iodine; only the synthetic element tennessine is heavier. All of its isotopes are radioactive and short-lived, so no visible amount has ever been gathered: a sample large enough to see would vaporize from the heat of its own radioactivity. Its melting point (about 302 °C, 575 K) and boiling point (about 337 °C, 610 K) are estimates.
Unlike the lighter halogens, astatine is expected to have metallic character, and it is often classed as a metalloid, as it is on this page.
Astatine has 85 electrons in six shells, arranged 2, 8, 18, 32, 18, 7, with the configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵.
About 40 isotopes are known, none of them stable. The longest-lived, astatine-210, has a half-life of 8.1 hours. Astatine-211, with a half-life of 7.2 hours, is the isotope used in medical research.
Mendeleev's periodic table left a gap below iodine for an element he called eka-iodine. Several claims to have found it in nature, under names such as alabamine (1931) and helvetium (1940), could not be confirmed.
In 1940 Dale R. Corson, Kenneth Ross MacKenzie and Emilio Segrè at the University of California, Berkeley, produced element 85 by bombarding bismuth-209 with alpha particles in a cyclotron, making astatine-211.
In 1943 Berta Karlik and Traude Bernert in Vienna detected astatine isotopes in the natural decay chains of uranium and thorium.
The discoverers named the element astatine in 1947, from the Greek astatos, meaning unstable.
Applications
Astatine is used only in research, mainly in nuclear medicine. Astatine-211 emits alpha particles that travel only a few cell widths, so when it is attached to antibodies or other molecules that seek out cancer cells, it can destroy them with little damage to nearby tissue. This targeted alpha therapy is being tested in clinical trials.
Chemists study astatine using only billionths of a gram at a time, with tracer techniques. These experiments test how relativistic effects change the chemistry of heavy elements and show that astatine behaves partly like iodine and partly like a metal.
None. Astatine is made only in tiny amounts in cyclotrons for research, and it decays within hours. Patients in clinical trials of astatine-211 therapy are the only people who meet it outside a laboratory.
Where it comes from
0 mg/kg of Earth's crust · more abundant than 0% of elements
Astatine forms naturally only as a brief stage in the radioactive decay chains of uranium and thorium, as isotopes such as astatine-215, 218 and 219. These natural isotopes have half-lives of a minute or less; only a few astatine isotopes, such as the synthetic astatine-210 and astatine-211, last hours. Estimates of how much astatine exists in Earth's crust at any moment range from less than one gram to a few tens of grams.
Astatine for research and medicine is made in cyclotrons by bombarding bismuth-209 with alpha particles, which produces astatine-211. It is separated by dry distillation and used within hours.
Handling
Astatine is radioactive. It has no stable isotope — every nucleus decays. Handling requires appropriate shielding and licensing.
Astatine is intensely radioactive. Its alpha particles are stopped by skin or paper, so the danger is internal: taken into the body, astatine collects in the thyroid gland, like iodine, and in other tissues, where its alpha radiation damages cells.
Astatine is handled only in nuclear medicine and radiochemistry laboratories, in tiny amounts, in shielded fume hoods or glove boxes, under radiation protection rules that control intake and contamination.
Quick answers
Astatine (symbol At) is element 85 on the periodic table, a metalloid in period 6, group 17. The rarest naturally occurring element on Earth — perhaps 25 grams exist in the entire crust at any moment. At room temperature it is a solid, and it is radioactive.
[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵, giving 6 occupied shells with the distribution 2, 8, 18, 32, 18, 7. Its outer shell holds 7 electrons, which is what sets its bonding behaviour.Astatine melts at 575 K (301.9 °C) and boils at 610 K (336.9 °C).
Astatine has no stable isotope, so it has no standard atomic weight. The figure quoted, 210, is the mass number of its longest-lived known isotope.
Astatine has no stable isotope. Its longest-lived known isotope, At-210, has 125 neutrons (mass number 210 minus atomic number 85) and a half-life of 8.1 hr. The classroom shortcut, rounded atomic mass (210) minus atomic number, gives the same 125.
Astatine is predicted to have a density of about 7 g/cm³, though no sample large enough to weigh has ever been made. Water is 1.0 g/cm³ for comparison.
Astatine has a Pauling electronegativity of 2.2. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.
Astatine was discovered in 1940 by Corson, MacKenzie & Segre. Origin of the name: Greek astatos, "unstable".
Astatine does not occur naturally on Earth in any meaningful quantity — it is made in a reactor or an accelerator.
Yes. Astatine has no stable isotope — every one of its nuclei decays. Trace amounts occur naturally as decay products of heavier elements.