Californium
Californium is a radioactive metallic chemical element with symbol Cf and atomic number 98. The element was first made in 1950 at the University of California Radiation Laboratory in Berkeley, by bombarding curium with alpha particles (helium-4 ions). It is an actinide element, the sixth transuranium element to be synthesized, and has the second-highest atomic mass of all the elements that have been produced in amounts large enough to see with the unaided eye (after einsteinium).
Identity
- Atomic mass
- [251] u
- Category
- Actinides
- Group
- 3
- Period
- 7
- Block
- f
- Oxidation States
- +2, +3, +4
- Phase (25 °C)
- Solid
- Appearance
- silvery
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f10
- Short configuration
- [Rn] 5f10 7s2
- Electrons per shell
- 2, 8, 18, 32, 28, 8, 2
- Electronegativity (Pauling)
- 1.30
- Electron affinity
- -97.3 kJ/mol
- 1st ionization energy
- 608 kJ/mol
Physical properties
- Melting point
- 899.9 °C (1173 K)
- Boiling point
- 1469.8 °C (1743 K)
- Density
- 15.1 g/cm³
- Molar heat capacity
- —
Chemical behavior of californium
Californium has ten electrons outside ([Rn] 5f¹⁰ 7s²) and behaves as +3, with +2 and +4 accessible in halides and oxides. Its electronegativity is low (1.30), and it is dense, malleable, and the most neutron-emitting actinide: one microgram of Cf-252 generates 2.3 million neutrons per second. It oxidizes in air and glows from its own heat.
Reactivity
4 Cf + 3 O₂ → 2 Cf₂O₃
In air it oxidizes quickly to the sesquioxide. The heat of spontaneous fission keeps the shielded sample warm to the touch.
2 Cf + 6 HCl → 2 CfCl₃ + 3 H₂
It dissolves in acids giving green californium(III) salts. The chloride is the route for purifying Cf-252 from berkelium parent.
Cf₂O₃ + 6 HF → 2 CfF₃ + 3 H₂O
With hydrofluoric acid it gives the trifluoride, a precursor for hermetically sealed sources and einsteinium blanks.
²⁵²Cf → fragments + 3.8 n
Cf-252 undergoes spontaneous fission (3% of its decays) releasing neutrons: the spark that starts reactors and scans petroleum.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Californium-252 source | ²⁵²Cf | Reactor startup and petroleum |
| Californium oxide | Cf₂O₃ | Hermetically sealed neutron sources |
| Californium trifluoride | CfF₃ | Purification and blanks |
| Californium oxalate | Cf₂(C₂O₄)₃ | Berkelium separation |
| Cf-249 blank | ²⁴⁹Cf | Synthesis of superheavies |
Electron shell distribution
View in the atom builderIsotopes
No californium isotope is stable. Californium-252 (²⁵²Cf, 2.6 years) emits neutrons and is the standard portable source. Californium-249 (²⁴⁹Cf, 351 years) is a blank for rutherfordium and seaborgium. Californium-251 has the longest half-life (898 years) but is extremely scarce.
Occurrence and production
Does not exist in nature: all californium is born in the high-flux reactor at Oak Ridge, when Bk-249 captures neutrons. Each yearly campaign yields milligrams. Only the USA and Russia isolate it: the most expensive material per gram after antimatter.
History and origin of the name
Thompson, Street, Ghiorso and Seaborg synthesized it in 1950 by bombarding curium with alphas at Berkeley, and named it californium after California: the state after Berkeley's berkelium. Isolated Cf-252 in 1952 revealed the intense spontaneous fission and made it a tool.
Applications and relevance of californium
Uses of the element
As Cf-252, microgram sources start reactors, record petroleum, and detect explosives and coal. One gram equals a startup reactor in a pocket shield.
Uses of its compounds
The sealed oxide in steel calibrates fuel-rod scanners and cement analyzers. Cf-249 bombarded with calcium produces rutherfordium and seaborgium.
Health, safety and environment
Biological role
It has no biological function and is extremely radiotoxic: neutrons and alphas that deposit in bone. One microgram inhaled is lethal in the medium term.
Safety
It is handled only at Oak Ridge and Dimitrovgrad, in cells with water shielding and paraffin against neutrons. Sources travel in certified containers with escort. It does not exist in consumer products.
Environment
There are no discharges: each microgram is traceable and waste vitrified. Its production consumes entire reactors, but a source avoids tons of startup equipment.
Frequently asked questions about Californium
What is californium-252 used for in nuclear reactor startup?
At the start of a nuclear reactor's operation, a neutron source is needed to initiate the chain reaction. Californium-252 emits neutrons spontaneously and can provide that initial spark. It is placed near the fuel as a sealed source, and its emission helps reach the reactivity level required for fission to be maintained in a controlled manner.
What is californium-252 used for in oil exploration?
In oil exploration, californium-252 is used as a neutron source in geophysical logging tools. The neutrons penetrate the subsurface and interact with the elements present in rocks and fluids. By analyzing the returning radiation, it is possible to estimate porosity, water or hydrocarbon saturation, and other properties of the reservoir. This helps decide where to drill.
What is the difference between californium-252 and californium-249?
Californium-252 is the most widely used isotope because it emits neutrons spontaneously and has a half-life of about 2.6 years, making it useful as a neutron source. Californium-249, on the other hand, has a much longer half-life, about 351 years, but does not emit neutrons spontaneously and is used mainly in research. The choice depends on the application.
What is californium's relationship with the periodic table?
Californium is element 98 and part of the actinides, the series of radioactive metallic elements from actinium to lawrencium. Its position in the periodic table places it between berkelium and einsteinium. As an actinide, it shares chemical properties with other elements in the series, but its isotopes are all radioactive and its production is extremely limited.
Why is californium-252 so expensive?
Its high cost is due to the difficulty of production, the small amount obtained per campaign, and the complexity of its separation and encapsulation. It is produced only in a few specialized reactors, and each gram requires years of irradiation and processing. Furthermore, its handling requires expensive radiological protection infrastructure. All this makes it one of the most expensive materials per gram that exist.
What is californium-252 used for in security and materials control?
Californium-252 is used in nuclear fuel rod scanners, explosive detection systems, and material composition analysis such as coal, cement, and minerals. Its neutron emission allows identification of elements present in a sample using techniques such as neutron activation analysis and neutron-induced gamma spectroscopy.
Why is californium called that?
Californium received its name in honor of the state of California, where it was discovered in 1950 by Stanley Thompson, Kenneth Street, Albert Ghiorso, and Glenn Seaborg. It was produced by bombarding curium with alpha particles in a cyclotron. The name recognizes the role of the University of California at Berkeley as a pioneering center for the synthesis of transuranic elements.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.