ActinideElement 98 · f-block

Californium (Cf)

Californium (Cf) is an actinide with atomic number 98; its most stable known isotope has mass number 251. At room temperature it is a solid that melts at 900 °C.

98CfCalifornium[251]

Atomic number

protons

Atomic weight

[251]

most stable isotope

Melting point

900 °C

1,173.15 K

Boiling point

—

Density

15.1 g/cm³

near room temperature

Electronegativity

1.3

Pauling scale

State at 25 °C

Solid

Discovered

1950

United States (Berkeley)

Californium at a glance

Identity

Symbol	Cf
Atomic number (Z)	98
Atomic weight	[251] mass number of the most stable isotope
Category	Actinide
Group	— (f-block: actinide series)
Period	7
Block	f-block
CAS number	7440-71-3

Atomic properties

Electron configuration	[Rn] 5f¹⁰ 7s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹⁰ 6s² 6p⁶ 7s²
Electrons per shell	2, 8, 18, 32, 28, 8, 2
Oxidation states	+3 Less common: +2, +4, +5
Electronegativity (Pauling)	1.3
First ionisation energy	6.282 eV (606.1 kJ/mol) 2nd: 12 eV · 3rd: 22.4 eV
Covalent radius	168 pm
Van der Waals radius	245 pm

Physical properties

State at 25 °C	Solid
Melting point	900 °C · 1,652 °F · 1,173.15 K
Density	15.1 g/cm³

Isotopes & abundance

Natural occurrence	Synthetic
Radioactive	Yes
Most stable isotope	²⁵¹Cf — half-life 898 years
Known isotopes	20

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

Californium electron configuration

Californium has 98 electrons. Written in short form, its ground-state configuration is [Rn] 5f¹⁰ 7s² — the electrons of the noble gas radon, plus 5f¹⁰ 7s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹⁰ 6s² 6p⁶ 7s².

5f↑↓↑↓↑↓↑↑↑↑

7s↑↓

Discovery and name

Discovered

1950 — Stanley G. Thompson, Kenneth Street Jr., Albert Ghiorso and Glenn T. Seaborg · United States (Berkeley).

Name

California.

Known for

Californium-252 is a strong neutron emitter used to start reactors, find oil-bearing rock layers and detect explosives.

Isotopes of californium

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

Isotope	Protons	Neutrons	Half-life
²⁵¹Cf	98	153	898 years

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

Californium 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
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
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

Frequently asked questions

6 answers

What is the atomic mass of californium?

Californium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [251], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does californium have?

A neutral californium atom has 98 protons and 98 electrons. Its most stable isotope, californium-251, has 153 neutrons.

What is the electron configuration of californium?

[Rn] 5f¹⁰ 7s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹⁰ 6s² 6p⁶ 7s²).

What are the melting and boiling points of californium?

Californium melts at 900 °C (1,652 °F).

Is californium a metal?

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

Who discovered californium?

1950 — Stanley G. Thompson, Kenneth Street Jr., Albert Ghiorso and Glenn T. Seaborg (United States (Berkeley)).