Astatine

Astatine (At) is a metalloid with atomic number 85. It has no stable isotope and no characteristic isotopic composition on Earth, so it has no standard atomic weight; its longest-lived isotope has mass number 210. Its electron configuration is [Xe] 4f14 5d10 6s2 6p5, which gives it 7 valence electrons.

Astatine key data

Symbol	At
Atomic number	85
Mass number (longest-lived isotope)	210
Protons	85
Neutrons (longest-lived isotope, astatine-210)	125
Electrons (neutral atom)	85
Valence electrons	7
Category	Metalloid
Period	6
Group	17
Block	p-block
Noble gas notation	[Xe] 4f14 5d10 6s2 6p5
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p5
Electrons per shell	2, 8, 18, 32, 18, 7

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

Properties of astatine

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown. The electron affinity is the measured value for astatine, from Wikipedia’s electron affinity data page, in place of PubChem’s 2.8 eV. The first ionisation energy is the value for astatine from the NIST Atomic Spectra Database, in place of PubChem’s 9.5 eV.

Predicted melting point	575 K (301.85 °C), 75th of 103 by melting point
Predicted density	7 g/cm³, 50th of 96 by density
Predicted state at room temperature	Solid
Electronegativity	2.2 (Pauling), joint 17th of 95 by electronegativity
Van der Waals radius	202 pm, joint 57th of 99 by atomic radius
First ionisation energy	9.3175 eV (899 kJ/mol), 22nd of 102 by first ionisation energy
Electron affinity	2.41578 eV (233.087 kJ/mol), 5th of 57 by electron affinity
Common oxidation states	+7, +5, +3, +1, −1
Discovered	1940

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Astatine electron configuration

The full electron configuration of astatine is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p5. With the noble-gas core collapsed it is [Xe] 4f14 5d10 6s2 6p5, and the outermost subshell is 6p5.

Bohr model of astatine (At): a nucleus and six shells holding 2, 8, 18, 32, 18 and 7 electrons from the inside out. — Bohr model of astatine (At). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of astatine (At), [Xe] 4f14 5d10 6s2 6p5, with the xenon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 6p. — Orbital diagram of astatine (At). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one astatine atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 210 / (6.02214076 × 10²³)
= 3.4871 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 210
= 2.8677 × 10²¹ atoms

Astatine has no standard atomic weight, because it has neither a stable isotope nor a characteristic isotopic composition on Earth. The figure used above is the mass number of astatine-210, so treat these as nominal.

Astatine on the periodic table

Astatine is in period 6, group 17, in the p-block.

The other elements of group 17 are fluorine, chlorine, bromine, iodine and tennessine.