---
title: "Fermium (Fm) — element 100"
description: "Fermium is element 100, symbol Fm, most stable isotope 257. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/fermium/
site: "At One Place"
---

# Fermium (Fm)

**Fermium — element 100: Fm**

[257] · Actinide · period 7

Fermium is the heaviest element that can be made by bombarding a target with neutrons in a reactor. Past it the chain stops: fermium-258 fissions spontaneously within about 370 microseconds of forming, so neutrons cannot build anything heavier and every element from mendelevium up has to be made by smashing nuclei together in an accelerator instead. That barrier is known as the fermium gap, and it is the reason element 101 onward were discovered one atom at a time.

## Properties

- **Atomic number:** 100
- **Symbol:** Fm
- **Mass number of most stable isotope:** [257]
- **Category:** Actinide
- **Period:** 7
- **Block:** f-block
- **Electron configuration:** [Rn] 7s² 5f¹²
- **Electrons per shell:** 2, 8, 18, 32, 30, 8, 2
- **Oxidation states:** +3, +2
- **Discovered:** 1952 by Albert Ghiorso and colleagues at Berkeley

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f¹²
- **Noble gas shorthand:** [Rn] 7s² 5f¹²
- **Shells:** K: 2 · L: 8 · M: 18 · N: 32 · O: 30 · P: 8 · Q: 2

## What it is used for

- Research only; no application outside the laboratory

## Hazards

- Intensely radioactive; has only ever existed in quantities of at most a few million atoms

> Melting point, boiling point, density and electronegativity are omitted. Published melting points disagree sharply — 1,800 K in some tables, 1,125 K in others — because both are extrapolations: fermium has never been prepared in a weighable quantity. The mass given is the mass number of fermium-257, the longest-lived at 100.5 days.

> Several physical properties are not listed for fermium because no bulk sample has ever existed. Only a handful of atoms have been produced, often with half-lives measured in seconds, so published melting points and densities for elements in this range are calculations rather than measurements. They are omitted rather than presented as observed values.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 98 | Californium | Cf | [251] |
| 99 | Einsteinium | Es | [252] |
| 100 | Fermium ← | Fm | [257] |
| 101 | Mendelevium | Md | [258] |
| 102 | Nobelium | No | [259] |

## Common questions

### What is the atomic number of fermium?

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

### What is the symbol for fermium?

Fm.

### What is the electron configuration of fermium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f¹², or [Rn] 7s² 5f¹² in noble-gas shorthand.

## Related pages

- [Californium (Cf)](https://atoneplace.net/element/californium/)
- [Einsteinium (Es)](https://atoneplace.net/element/einsteinium/)
- [Mendelevium (Md)](https://atoneplace.net/element/mendelevium/)
- [Nobelium (No)](https://atoneplace.net/element/nobelium/)

## Sources

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