---
title: "Francium (Fr) — element 87"
description: "Francium is element 87, symbol Fr, most stable isotope 223. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/francium/
site: "At One Place"
---

# Francium (Fr)

**Francium — element 87: Fr**

[223] · Alkali metal · period 7, group 1

Francium is the least stable of the first 101 elements: its longest-lived isotope, francium-223, has a half-life of 22 minutes, so any sample destroys itself before it can be studied conventionally. There is thought to be no more than about 30 grams of it in the entire Earth's crust at any instant, and nobody has ever seen a bulk sample or ever will — the largest quantity ever assembled was roughly 300,000 atoms held in a magneto-optical trap at Stony Brook. It was the last element to be discovered in nature rather than made, found by Marguerite Perey at the Curie Institute in 1939.

## Properties

- **Atomic number:** 87
- **Symbol:** Fr
- **Mass number of most stable isotope:** [223]
- **Category:** Alkali metal
- **Group:** 1
- **Period:** 7
- **Block:** s-block
- **Electron configuration:** [Rn] 7s¹
- **Electrons per shell:** 2, 8, 18, 32, 18, 8, 1
- **Electronegativity:** 0.7 (Pauling)
- **Oxidation states:** +1
- **Discovered:** 1939 by Marguerite Perey

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s¹
- **Noble gas shorthand:** [Rn] 7s¹
- **Shells:** K: 2 · L: 8 · M: 18 · N: 32 · O: 18 · P: 8 · Q: 1

## What it is used for

- Atom-trap experiments testing the weak nuclear force and searching for physics beyond the Standard Model

## Hazards

- Intensely radioactive; a visible quantity would destroy itself and anything near it by its own decay heat

> Melting point, boiling point and density are omitted deliberately: the commonly quoted 27 °C melting point is an extrapolation from the other alkali metals, not a measurement, because a weighable sample of francium has never existed. The electronegativity of 0.7 is a calculated value; measurements on trapped atoms suggest francium is slightly less electropositive than caesium, so caesium, not francium, is the most electropositive element.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 85 | Astatine | At | [210] |
| 86 | Radon | Rn | [222] |
| 87 | Francium ← | Fr | [223] |
| 88 | Radium | Ra | [226] |
| 89 | Actinium | Ac | [227] |

## Group 1

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Francium sits in group 1 with hydrogen, lithium, sodium, potassium, rubidium, cesium.

## Common questions

### What is the atomic number of francium?

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

### What is the symbol for francium?

Fr.

### What is the electron configuration of francium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s¹, or [Rn] 7s¹ in noble-gas shorthand.

## Related pages

- [Astatine (At)](https://atoneplace.net/element/astatine/)
- [Radon (Rn)](https://atoneplace.net/element/radon/)
- [Radium (Ra)](https://atoneplace.net/element/radium/)
- [Actinium (Ac)](https://atoneplace.net/element/actinium/)
- [Hydrogen (H)](https://atoneplace.net/element/hydrogen/)
- [Lithium (Li)](https://atoneplace.net/element/lithium/)
- [Sodium (Na)](https://atoneplace.net/element/sodium/)
- [Potassium (K)](https://atoneplace.net/element/potassium/)
- [Rubidium (Rb)](https://atoneplace.net/element/rubidium/)
- [Cesium (Cs)](https://atoneplace.net/element/cesium/)

## Sources

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