ActinideElement 96 · f-block

Curium (Cm)

Curium (Cm) is an actinide with atomic number 96; its most stable known isotope has mass number 247. At room temperature it is a solid that melts at 1,345 °C.

96CmCurium[247]

Atomic number

protons

Atomic weight

[247]

most stable isotope

Melting point

1,345 °C

1,618.15 K

Boiling point

—

Density

13.51 g/cm³

near room temperature

Electronegativity

1.3

Pauling scale

State at 25 °C

Solid

Discovered

1944

United States (Berkeley)

Curium at a glance

Identity

Symbol	Cm
Atomic number (Z)	96
Atomic weight	[247] mass number of the most stable isotope
Category	Actinide
Group	— (f-block: actinide series)
Period	7
Block	f-block
CAS number	7440-51-9

Atomic properties

Electron configuration	[Rn] 5f⁷ 6d¹ 7s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f⁷ 6s² 6p⁶ 6d¹ 7s²
Electrons per shell	2, 8, 18, 32, 25, 9, 2
Oxidation states	+3 Less common: +4, +5, +6
Electronegativity (Pauling)	1.3
First ionisation energy	5.992 eV (578.2 kJ/mol) 2nd: 12.4 eV · 3rd: 20.1 eV
Covalent radius	166 pm
Van der Waals radius	245 pm

Physical properties

State at 25 °C	Solid
Melting point	1,345 °C · 2,453 °F · 1,618.15 K
Density	13.51 g/cm³

Isotopes & abundance

Natural occurrence	Synthetic
Radioactive	Yes
Most stable isotope	²⁴⁷Cm — half-life 15.6 million years
Known isotopes	22

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

Curium electron configuration

Curium has 96 electrons. Written in short form, its ground-state configuration is [Rn] 5f⁷ 6d¹ 7s² — the electrons of the noble gas radon, plus 5f⁷ 6d¹ 7s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f⁷ 6s² 6p⁶ 6d¹ 7s².

An exception to the filling rule. The simple aufbau (Madelung) order predicts 5f⁸, 6d⁰, but curium actually has 5f⁷, 6d¹. Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.

5f↑↑↑↑↑↑↑

6d↑

7s↑↓

Discovery and name

Discovered

1944 — Glenn T. Seaborg, Ralph A. James and Albert Ghiorso · United States (Berkeley).

Name

Marie and Pierre Curie.

Known for

Curium-244 has powered alpha-particle X-ray spectrometers on Mars rovers. Named after Marie and Pierre Curie.

Isotopes of curium

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

Isotope	Protons	Neutrons	Half-life
²⁴⁷Cm	96	151	15.6 million years

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

Curium 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
Neptunium	93	[237]	644 °C	—	20.2 g/cm³	1.36
Plutonium	94	[244]	640 °C	3,228 °C	19.7 g/cm³	1.28
Americium	95	[243]	1,176 °C	—	12 g/cm³	1.3
Curium	96	[247]	1,345 °C	—	13.51 g/cm³	1.3
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

Frequently asked questions

6 answers

What is the atomic mass of curium?

Curium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [247], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does curium have?

A neutral curium atom has 96 protons and 96 electrons. Its most stable isotope, curium-247, has 151 neutrons.

What is the electron configuration of curium?

[Rn] 5f⁷ 6d¹ 7s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f⁷ 6s² 6p⁶ 6d¹ 7s²).

What are the melting and boiling points of curium?

Curium melts at 1,345 °C (2,453 °F).

Is curium a metal?

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

Who discovered curium?

1944 — Glenn T. Seaborg, Ralph A. James and Albert Ghiorso (United States (Berkeley)).