Astatine

group 17 · period 6 · p-block · metalloid

Identity

Name and symbol: Astatine, At
Atomic number: 85 protons
Position: group 17 · period 6 · p-block · metalloid
CAS number: 7440-68-8

Atomic structure

Atomic mass: 210 u
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁵
[Xe] 6s²⁴f¹⁴⁵d¹⁰⁶p⁵
Electrons per shell: 2, 8, 18, 32, 18, 7
Valence electrons: 7 outer shell

nuclide	half-life	decay
²¹⁰At	8.1 h	β+=99.825±2%; α=0.175±2%
²¹¹At	7.214 h	ε=58.20±0.8%; α=41.80±0.8%
²⁰⁹At	5.42 h	β+=96.1±0.4%; α=3.9±0.4%
²⁰⁷At	1.81 h	β+≈90%; α≈10%

no stable isotope; the longest-lived nuclides

Physical properties

State at room temperature: Solid
Melting point: 575 K (301.85 °C)
Boiling point: 337 K (63.85 °C)
Density: not in sources
Appearance: unknown, probably metallic
Thermal conductivity: not in sources
Electrical resistivity: not in sources
Electrical conductivity: not in sources
Crystal structure: face-centered cubic
Molar heat capacity: not in sources

Chemical properties

Oxidation states: 7, 5, 3, 1, -1
Electronegativity: 2.2 (Pauling Scale)
Ionisation energy: 9.5 eV
1st 899 kJ/mol
Electron affinity: 2.8 eV
Atomic radius: empirical 150, covalent 150, van der Waals 202 pm
Ionic radius: At⁷⁺ 62 pm

Reactivity: The heaviest natural halogen and the rarest natural element, known only from tracer solutions below about 10^-10 mol per litre: it behaves mostly like iodine (the least reactive natural halogen, odd oxidation states from -1 to +7) but also shows metallic traits, plating on a cathode, coprecipitating with metal sulfides and forming an At+ cation that resembles silver(I).
with water: Not known as a bulk reaction; in aqueous tracer solutions astatine exists as astatide At-, as At0 or At+, or as the oxyanions AtO- and AtO(OH)2-, depending on the oxidant present.
with oxygen, air: Not known for elemental astatine and oxygen gas; the oxygen species AtO- and AtO+ have been made only in solution with oxidants such as bromine or persulfate in perchloric acid.
with acids: Not known in the usual sense: dilute nitric acid oxidises astatide to At0 or At+, from which silver(I) precipitates astatine only partly as AgAt, and in hydrochloric acid astatine coprecipitates with metal sulfides.
with halogens: Forms interhalogen molecules with the lighter halogens in solution: AtI from iodine and iodide, AtBr from iodine monobromide and bromide, and AtCl (or AtOCl) from chloride and oxidised astatine in nitric acid.

Typical compounds

HAt hydrogen astatide easily oxidised; hydrogen may carry the negative charge
AgAt silver astatide partly precipitated from acid tracer solutions
NaAt sodium astatide one of the few reported metal astatides
AtI astatine monoiodide interhalogen made in iodine and iodide solution
C₆H₅At astatobenzene astatine replacing hydrogen on benzene

Occurrence, production and use

Crustal abundance: Not Applicable
Oceanic abundance: Not Applicable

Occurrence and sources

not found in weighable amounts; made artificially produced in accelerators and reactors from bismuth

Extraction, production: Alpha bombardment of bismuth (1940 discovery route)
RSC states the element was made by bombarding bismuth with alpha particles and elsewhere that astatine-211 is made by neutron bombardment of bismuth-200; the second statement is inconsistent (no bismuth-200 target exists) and is not repeated here

Uses: Due to the small amounts produced and its short half-life, there are currently no uses for astatine outside of basic scientific research.

Safety, toxicity: not in sources

Discovery and name

Discovered by: Dale R. Corson, Kenneth Ross MacKenzie, Emilio Segrè
Discovered: 1940
First isolated: not in sources
Named by: not in sources
Origin of the name: Ancient Greek ástatos (ἄστατος) unstable

The "time of flight" mass spectrometer has been used to confirm that this highly radioactive halogen behaves chemically very much like other halogens, particularly iodine. Astatine is said to be more metallic than iodine, and, like iodine, it probably accumulates in the thyroid gland. Workers at the Brookhaven National Laboratory have recently used reactive scattering in crossed molecular beams to identify and measure elementary reactions involving astatine.

Conventions

Valence electrons are counted by the usual convention: the outer shell for s- and p-block elements, ns and (n-1)d for the d-block, ns, (n-1)d and (n-2)f for the f-block. Lanthanides and actinides are placed in the f-block with no group number. Electrical conductivity is the reciprocal of the printed resistivity. Ionic radii are Shannon effective radii, six-coordinate unless noted. Where a field reads “not in sources” the value was not found; it is a gap, not a zero. Regulatory limits are the published values and change often, so check the standard in force at your site and the numbers written into your own permit before you design to them.

Data

Element records, isotopes, radii and the descriptive text come from PubChem (NCBI), the Los Alamos National Laboratory periodic table, IUPAC CIAAW and the IAEA Atomic Mass Data Center. Appearance, thermal conductivity, electrical resistivity, crystal structure, discovery and the origin of each name come from Wikipedia and Periodic-Table-JSON, used under CC BY-SA 4.0. Hazard classifications come from the ECHA C&L inventory via PubChem. Ionic radii follow R. D. Shannon (1976). The water chapters cite their own sources at the foot of each entry, and are written to the level of Snoeyink and Jenkins, Stumm and Morgan, MWH's Water Treatment and Metcalf and Eddy.