HalogenElement 85 · p-block

Astatine (At)

Astatine (At) is a halogen with atomic number 85; its most stable known isotope has mass number 210. At room temperature it is a solid that melts at 302 °C (estimated).

85AtAstatine[210]

Atomic number

protons

Atomic weight

[210]

most stable isotope

Melting point

302 °C

estimated

Boiling point

—

Density

7 g/cm³

predicted

Electronegativity

2.2

Pauling scale

State at 25 °C

Solid

Discovered

1940

United States (Berkeley)

Astatine at a glance

Identity

Symbol	At
Atomic number (Z)	85
Atomic weight	[210] mass number of the most stable isotope
Category	Halogen
Group	17 — halogens
Period	6
Block	p-block
CAS number	7440-68-8

Atomic properties

Electron configuration	[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵ Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁵
Electrons per shell	2, 8, 18, 32, 18, 7
Oxidation states	-1, +1 Less common: +3, +5, +7
Electronegativity (Pauling)	2.2
First ionisation energy	9.318 eV (899 kJ/mol) 2nd: 17.88 eV · 3rd: 26.58 eV
Electron affinity	2.8 eV
Covalent radius	147 pm
Van der Waals radius	202 pm

Physical properties

State at 25 °C	Solid
Melting point	302 °C · 575.6 °F · 575.15 K estimated
Density	7 g/cm³ (predicted)

Isotopes & abundance

Natural occurrence	From radioactive decay
Radioactive	Yes
Most stable isotope	²¹⁰At — half-life 8.1 h
Known isotopes	39

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Astatine electron configuration

Astatine has 85 electrons. Written in short form, its ground-state configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵ — the electrons of the noble gas xenon, plus 4f¹⁴ 5d¹⁰ 6s² 6p⁵.Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁵.

As a member of group 17 it has 7 valence electrons in its outer shell (shell 6), which largely decide its chemistry.

4f↑↓↑↓↑↓↑↓↑↓↑↓↑↓

5d↑↓↑↓↑↓↑↓↑↓

6s↑↓

6p↑↓↑↓↑

Discovery and name

Discovered

1940 — Dale R. Corson, Kenneth Ross MacKenzie and Emilio Segrè · United States (Berkeley).

Name

Greek astatos, unstable.

Known for

The rarest naturally occurring element — less than a gram exists in Earth's crust at any time. Astatine-211 is studied for targeted cancer therapy.

Isotopes of astatine

Astatine has no stable isotopes. Its longest-lived known isotope is:

Isotope	Protons	Neutrons	Half-life
²¹⁰At	85	125	8.1 h

39 isotopes of astatine are known in our data set (half-lives as published in nuclear data tables).

Astatine compared with group 17

The other elements in group 17 (halogens), from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Fluorine	9	18.998	-220 °C	-188 °C	1.696 g/L	3.98
Chlorine	17	35.45	-101 °C	-34 °C	3.2 g/L	3.16
Bromine	35	79.904	-7.2 °C	59 °C	3.103 g/cm³	2.96
Iodine	53	126.90	114 °C	184 °C	4.933 g/cm³	2.66
Astatine	85	[210]	302 °C (est.)	—	7 g/cm³ (pred.)	2.2
Tennessine	117	[294]	—	—	7.2 g/cm³ (pred.)	—

Frequently asked questions

6 answers

What is the atomic mass of astatine?

Astatine has no stable isotopes, so it has no standard atomic weight. Periodic tables show [210], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does astatine have?

A neutral astatine atom has 85 protons and 85 electrons. Its most stable isotope, astatine-210, has 125 neutrons.

What is the electron configuration of astatine?

[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵ (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁵).

What are the melting and boiling points of astatine?

Astatine melts at 302 °C (575.6 °F) (estimated).

Is astatine a metal?

Astatine is a nonmetal (a halogen). It is in group 17, period 6, and is a solid at room temperature.

Who discovered astatine?

1940 — Dale R. Corson, Kenneth Ross MacKenzie and Emilio Segrè (United States (Berkeley)).