---
title: "Astatine (At) — element 85"
description: "Astatine is element 85, symbol At, most stable isotope 210. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/astatine/
site: "At One Place"
---

# Astatine (At)

**Astatine — element 85: At**

[210] · Halogen · period 6, group 17

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

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 83 | Bismuth | Bi | 208.98 |
| 84 | Polonium | Po | [209] |
| 85 | Astatine ← | At | [210] |
| 86 | Radon | Rn | [222] |
| 87 | Francium | Fr | [223] |

## 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.

## Common questions

### What is the atomic number of astatine?

85. That is the number of protons in its nucleus, which is what defines the element.

### What is the symbol for astatine?

At.

### What is the electron configuration of astatine?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁵, or [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁵ in noble-gas shorthand.

## Related pages

- [Bismuth (Bi)](https://atoneplace.net/element/bismuth/)
- [Polonium (Po)](https://atoneplace.net/element/polonium/)
- [Radon (Rn)](https://atoneplace.net/element/radon/)
- [Francium (Fr)](https://atoneplace.net/element/francium/)
- [Fluorine (F)](https://atoneplace.net/element/fluorine/)
- [Chlorine (Cl)](https://atoneplace.net/element/chlorine/)
- [Bromine (Br)](https://atoneplace.net/element/bromine/)
- [Iodine (I)](https://atoneplace.net/element/iodine/)
- [Tennessine (Ts)](https://atoneplace.net/element/tennessine/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)