Atomic number 85

Astatine

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85 electrons in 6 shells: 2, 8, 18, 32, 18, 7.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Halogen
Period: 6
Group: 17
Block: p
Atomic mass: 209.98715 Da (longest-lived isotope)
Electron configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵
Phase at 25 °C: solid
Melting point: 302 °C
Boiling point: 337 °C
Density: 7 g/cm³
Electronegativity (Pauling): 2.2
Discovered: 1940, D. R. Corson, K. R. MacKenzie and E. Segrè

Neighbours in the table

Astatine takes its name from astatos, Greek for unstable, in Los Alamos’s account. Astatine-210 is half gone after 8.1 hours in both Jefferson Lab’s figure and the evaluation of the International Atomic Energy Agency (IAEA). PubChem gives the mass of astatine’s longest-lived isotope, astatine-210, 209.98715 Da. It decays to polonium-210 by electron capture, the nucleus taking in one of its own electrons, or to bismuth-206 by alpha emission. IAEA puts 99.825 per cent of its decays through electron capture and positron emission and 0.175 per cent through alpha emission.

Group 17 places astatine beneath iodine, and its configuration, [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵, is spread across six shells holding 2, 8, 18, 32, 18 and 7 electrons. Its seven outer electrons, 6s² 6p⁵, leave it one electron short of a full 6p subshell. Los Alamos lists oxidation states of 7, 5, 3, 1 and −1, bookkeeping numbers that assume every bond is ionic.

Astatine behaves chemically much like the other halogens, iodine above all, according to Los Alamos. Astatine is reported to be more metallic than iodine, in Los Alamos’s account, and Jefferson Lab classes it as a semi-metal. Its Pauling electronegativity, an atom’s pull on the electrons of a bond it shares, is 2.2, right at the boundary under which Chem1 expects metallic behaviour.

Where it is found

Astatine exists in nature only in passing, as a product of radioactive decay. Small amounts form as uranium and thorium break down, in Jefferson Lab’s account. Los Alamos adds traces of astatine-217 kept in equilibrium with uranium-233 and neptunium-239, which naturally produced neutrons create from thorium and uranium. At any one time the whole crust holds less than 30 grams of astatine by Jefferson Lab’s estimate, and less than an ounce by Los Alamos’s.

That scarcity is why astatine is made only when a study needs it, Jefferson Lab explains. Energetic alpha particles fired at bismuth yield astatine-209, astatine-210 and astatine-211, the relatively long-lived trio, in Los Alamos’s description. The astatine is then separated from the bismuth target, Los Alamos adds. Astatine-209 has a half-life of 5.42 hours, according to PubChem. Jefferson Lab puts the total amount of astatine ever produced at 0.05 micrograms.

Why it matters

Astatine was first made in 1940 at the University of California, and Los Alamos credits D. R. Corson, K. R. MacKenzie and E. Segrè. The team aimed alpha particles, sped up in a cyclotron, at bismuth-209, Jefferson Lab records, and the reaction gave astatine-211 and two free neutrons. IAEA’s data date the discovery of astatine-211 to 1940 and that of astatine-210, the longer-lived isotope, to 1949.

Astatine-211 has a small place in medicine, according to IUPAC, chemistry’s international union. It collects in the thyroid, and it is sometimes chosen for treating an overactive thyroid, hyperthyroidism, and thyroid cancer. It is preferred to iodine made radioactive, the other treatment, because its particles carry more energy, in IUPAC’s account. A carborane, a cluster of boron, carbon and hydrogen atoms, carrying astatine-211 and called the Venus Flytrap Cluster, has served as a robust radiotherapy drug.

That use has limits. Astatine has shown a tendency to induce tumours, IUPAC adds, so it is used sparingly. Astatine-211’s half-life is 7.214 hours in IAEA’s evaluation. IAEA’s data send 58.2 per cent of its decays through electron capture and 41.8 per cent through alpha emission. Astatine has no uses outside basic research, in Jefferson Lab’s assessment, because so little is made and its half-life is short.

Properties

Properties of astatine Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	85
Atomic mass	209.98715 Da	Da	PubChem, longest-lived isotope
Melting point	302 °C	°C	Los Alamos: 302 °C
Boiling point	337 °C	°C	Los Alamos: 337 °C
Density	7 g/cm³	g/cm³
Electronegativity (Pauling)	2.2
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁵
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Astatine is a highly radioactive halogen, in Los Alamos’s words, and its hazard is ionising radiation, as Haz-Map’s summary in PubChem’s safety record states. Astatine-211 emits alpha particles in a large share of its decays, in IAEA’s data, and like iodine the element probably gathers in the thyroid gland, Los Alamos says. The hazard concerns the laboratories that make astatine and, where IUPAC’s thyroid treatment is used, the patients who receive it. PubChem’s Laboratory Chemical Safety Summary for astatine is the reference sheet.

Questions

How much astatine is there on Earth?

Very little at any moment. Jefferson Lab estimates that the crust holds less than 30 grams in total, and Los Alamos less than an ounce. PubChem counts no stable isotope, and the atoms present come from the decay of uranium and thorium. Jefferson Lab also puts everything ever produced in laboratories at 0.05 micrograms, and astatine is made only when a study calls for it.

What is the half-life of astatine?

It depends on the isotope, and no astatine isotope has a half-life as long as a day. Astatine-210 has the longest, 8.1 hours in both Jefferson Lab's figure and IAEA's evaluation. Astatine-211 follows, at 7.214 hours in IAEA's evaluation. In IAEA's data, 58.2 per cent of its decays go by electron capture and 41.8 per cent by alpha emission. PubChem lists astatine-209 at 5.42 hours.

Is astatine a metal or a halogen?

It sits in group 17 with the halogens and reacts much like iodine, Los Alamos says, which also reports claims that it is more metallic than iodine; Jefferson Lab classes it as a semi-metal. Its ionisation energy, the energy needed to remove its outermost electron, is 9.5 eV in Jefferson Lab's figure and 9.31751 eV in NIST's. On the Allen scale of electronegativity PubChem lists 2.39.

Is astatine used in medicine?

Rarely, and only astatine-211. The isotope is known to collect in the thyroid, according to IUPAC, and is occasionally preferred for treating an overactive thyroid and thyroid cancer, though its tendency to induce tumours limits that use. Astatine probably gathers in the thyroid as iodine does, in Los Alamos's wording. It has no uses beyond basic research in Jefferson Lab's view, given how little is made and how short its half-life is.