Astatine (At)

Astatine is the rarest naturally occurring element on Earth: the entire crust is estimated to hold less than 30 grams at any moment, all of it fleeting decay product. Nobody has ever seen a visible quantity and nobody ever will, because a weighable sample would vaporise itself from its own radioactive heating before it could be looked at. Its bulk appearance and melting point are extrapolations from iodine and from the periodic trend, and modern work suggests astatine may behave more like a metal than like a halogen.

Properties

Atomic number

85

Symbol

At

Mass number of most stable isotope

[210]

Category

Halogen

Group

17

Period

6

Block

p-block

Electron configuration

[Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁵

Electrons per shell

2, 8, 18, 32, 18, 7

Electronegativity

2.2 (Pauling)

Oxidation states

-1, +1, +3, +5, +7

Discovered

1940 by Dale Corson, Kenneth MacKenzie and Emilio Segrè

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁵

Noble gas shorthand

[Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁵

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 18 · P: 7

What it is used for

• Astatine-211 in experimental targeted alpha therapy for cancer
• Tracer studies of halogen chemistry

Hazards

• Intensely radioactive; concentrates in the thyroid like iodine

Melting point, boiling point and density are omitted deliberately: every published figure for them is an extrapolation, because a bulk sample of astatine has never existed. The longest-lived isotope, astatine-210, has a half-life of 8.1 hours. The electronegativity quoted is a calculated value, not a measured one.

Neighbours in the table

Group 17

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Astatine sits in group 17 with fluorine, chlorine, bromine, iodine, tennessine.