ActinideElement 100 · f-block

Fermium (Fm)

Fermium (Fm) is an actinide with atomic number 100; its most stable known isotope has mass number 257. At room temperature it is a solid that melts at 1,527 °C (estimated).

100FmFermium[257]

Atomic number

100

protons

Atomic weight

[257]

most stable isotope

Melting point

1,527 °C

estimated

Boiling point

—

Density

9.7 g/cm³

predicted

Electronegativity

1.3

Pauling scale

State at 25 °C

Solid

Discovered

1952

United States

Fermium at a glance

Identity

Symbol	Fm
Atomic number (Z)	100
Atomic weight	[257] mass number of the most stable isotope
Category	Actinide
Group	— (f-block: actinide series)
Period	7
Block	f-block
CAS number	7440-72-4

Atomic properties

Electron configuration	[Rn] 5f¹² 7s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹² 6s² 6p⁶ 7s²
Electrons per shell	2, 8, 18, 32, 30, 8, 2
Oxidation states	+3 Less common: +2
Electronegativity (Pauling)	1.3
First ionisation energy	6.5 eV (627.2 kJ/mol) 2nd: 12.4 eV · 3rd: 23.2 eV
Covalent radius	167 pm
Van der Waals radius	245 pm

Physical properties

State at 25 °C	Solid
Melting point	1,527 °C · 2,780.6 °F · 1,800.15 K estimated
Density	9.7 g/cm³ (predicted)

Isotopes & abundance

Natural occurrence	Synthetic
Radioactive	Yes
Most stable isotope	²⁵⁷Fm — half-life 100.5 days
Known isotopes	20

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

Fermium electron configuration

Fermium has 100 electrons. Written in short form, its ground-state configuration is [Rn] 5f¹² 7s² — the electrons of the noble gas radon, plus 5f¹² 7s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹² 6s² 6p⁶ 7s².

5f↑↓↑↓↑↓↑↓↑↓↑↑

7s↑↓

Discovery and name

Discovered

1952 — Albert Ghiorso and co-workers (debris of the 'Ivy Mike' test) · United States.

Name

Enrico Fermi.

Known for

Also found in hydrogen-bomb debris; the heaviest element that can be made by neutron capture in reactors.

Isotopes of fermium

Fermium has no stable isotopes and does not occur naturally in significant amounts. Its longest-lived known isotope is:

Isotope	Protons	Neutrons	Half-life
²⁵⁷Fm	100	157	100.5 days

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

Fermium and its neighbours in the actinide series

Neighbouring elements in the same f-block series:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Berkelium	97	[247]	986 °C	—	14.78 g/cm³	1.3
Californium	98	[251]	900 °C	—	15.1 g/cm³	1.3
Einsteinium	99	[252]	860 °C (est.)	—	8.84 g/cm³ (pred.)	1.3
Fermium	100	[257]	1,527 °C (est.)	—	9.7 g/cm³ (pred.)	1.3
Mendelevium	101	[258]	827 °C (est.)	—	10.3 g/cm³ (pred.)	1.3
Nobelium	102	[259]	827 °C (est.)	—	9.9 g/cm³ (pred.)	1.3
Lawrencium	103	[266]	1,627 °C (est.)	—	15.6 g/cm³ (pred.)	1.3

Frequently asked questions

6 answers

What is the atomic mass of fermium?

Fermium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [257], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does fermium have?

A neutral fermium atom has 100 protons and 100 electrons. Its most stable isotope, fermium-257, has 157 neutrons.

What is the electron configuration of fermium?

[Rn] 5f¹² 7s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹² 6s² 6p⁶ 7s²).

What are the melting and boiling points of fermium?

Fermium melts at 1,527 °C (2,780.6 °F) (estimated).

Is fermium a metal?

Fermium is a metal. It is in the f-block, period 7, and is a solid at room temperature.

Who discovered fermium?

1952 — Albert Ghiorso and co-workers (debris of the 'Ivy Mike' test) (United States).