Astatine

The rarest naturally occurring element, a halogen so radioactive that no one has ever seen a visible piece of it.

At a glance

Symbol: At
Atomic number: 85
Mass number of its longest-lived isotope: [210]
Group: 17
Period: 6
Family: metalloid
Electron configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵
Valence electrons: 7
Electronegativity: 2.20
Common ions: At⁻, and positive states such as At⁺
Isotopes: none are stable; ²¹⁰At lasts longest, with a half-life of about 8 hours

On the table

Period 6, group 17. The other metalloids are tinted, and every cell opens its own article.

Electrons and bonding

Astatine has 85 electrons, one for every proton, so the atom carries no charge. Below are the same 85 electrons written two ways.

Simple form: electrons per shell

shell 12shell 28shell 318shell 432shell 518shell 67

Counting outward from the nucleus: 2, 8, 18, 32, 18, 7. The outer shell is picked out.

Orbital form: electrons per subshell

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

Short form: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵, where [Xe] stands for the 54 electrons xenon already has.

That list runs by shell. In the order the subshells fill it reads [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁵, because a subshell fills by its energy, not by its shell number.

4f¹⁴↑↓↑↓↑↓↑↓↑↓↑↓↑↓

5d¹⁰↑↓↑↓↑↓↑↓↑↓

6s²↑↓

6p⁵↑↓↑↓↑

The boxes hold the electrons beyond [Xe]. Each box is one orbital and takes two electrons at most, spinning opposite ways. They spread out singly before they pair up.

How to read the two

The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 4 here is 4s² 4p⁶ 4d¹⁰ 4f¹⁴, and 2 + 6 + 10 + 14 is 32.

Astatine sits in period 6 and has 6 shells. The period number counts the shells.

What that means for bonding

The outer shell holds 7 electrons. Those are the ones astatine can lose, gain or share, so they are the ones that decide what it bonds with. For a main-group element the group number gives that count: group 17 means 7.

Seven valence electrons, like the other halogens, but it is the most metal-like of them. Only trace amounts have ever been studied.

Properties

State at 25 °C: solid
What it looks like: never seen; a visible sample would vaporize itself with the heat of its own radioactivity
Melting point: 302 °C (estimated)
Boiling point: not known
Density: not measured

Where it is found

Formed briefly in uranium and thorium ores as they decay. Estimates of how much exists in all of Earth's crust at any moment range from under a gram to about an ounce.

Discovery and name

Dale Corson, Kenneth MacKenzie and Emilio Segrè made it in 1940 at Berkeley by bombarding bismuth with alpha particles. It was named from the Greek astatos, unstable.

What it is used for

Astatine-211 is being tested in targeted alpha therapy, attached to molecules that carry it to cancer cells. Its short half-life keeps it in research labs.

In compounds

HAthydrogen astatideAtIastatine monoiodide

Handle with care

Intensely radioactive, handled only in tiny amounts in specialized labs.