Atomic number 98

Californium

Cf
+ phase
− phase

Shells Orbitals 5f 7s

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98 electrons in 7 shells: 2, 8, 18, 32, 28, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Actinide
Period: 7
Block: f
Atomic mass: 251.07959 Da (longest-lived isotope)
Electron configuration: [Rn] 5f¹⁰ 7s²
Phase at 25 °C: solid
Melting point: 900 °C
Boiling point: 1470 °C
Electronegativity (Pauling): 1.3
Discovered: 1950, Stanley G. Thompson, Glenn T. Seaborg, Kenneth Street, Jr. and Albert Ghiorso

Neighbours in the table

Bk Es

In 1950 Stanley G. Thompson, Glenn T. Seaborg, Kenneth Street, Jr. and Albert Ghiorso produced element 98 at Berkeley, according to Jefferson Lab. They named it for the state of California and its university, Los Alamos records. Their Berkeley 60-inch cyclotron fired helium ions at microgram amounts of curium-242. Jefferson Lab and IUPAC (the International Union of Pure and Applied Chemistry) give the product as californium-245, whose half-life Jefferson Lab puts at about 45 minutes; Los Alamos gives californium-244.

A chemical forecast guided the search. Dysprosium, californium’s lanthanide homologue (the element in the matching position one row up), keeps a stable +3 state in water. The Berkeley group therefore expected californium to hold a stable +3 state too, Los Alamos explains. The forecast proved right, and it allowed a chromatographic separation, a sorting of dissolved ions, that pulled californium away from the other actinides and identified it without doubt.

Counted shell by shell, californium’s electrons run 2, 8, 18, 32, 28, 8 and 2, the configuration [Rn] 5f¹⁰ 7s². Its 5f electrons are shielded from the valence electrons more than in the lighter actinides, so in solid compounds californium behaves like the lanthanides, Los Alamos says. Solid compounds show +2, +3 and +4, while in solution +3 is the most stable state and a +5 state remains doubtful.

Where it is found

No californium exists naturally in the crust of the Earth, IUPAC states, and none of its isotopes is stable. PubChem gives the mass of its longest-lived isotope, californium-251, 251.080 Da. It halves in 898 years in the IAEA’s (International Atomic Energy Agency) evaluation and 900 years in HSDB (Hazardous Substances Data Bank). About 100 per cent of its decays are alpha emissions in IAEA’s data, and spontaneous fission, the nucleus splitting unprompted, also occurs.

Certain supernovae, exploding stars, may produce californium, Los Alamos reports. The shape of their light curves, the record of a star’s brightness over time, seemed to match the decay of californium-254. That idea has been questioned. Californium-254 halves in 55 days for Los Alamos and 60.5 days for the Atomic Mass Data Center. The centre gives 99.69 per cent spontaneous fission and 0.31 per cent alpha.

On Earth, californium is made. Californium-249 forms by beta decay, electron emission from the nucleus, of berkelium-249, and heavier isotopes build up under intense neutron irradiation in reactors. Oak Ridge National Laboratory’s High Flux Isotope Reactor and high-flux reactors at Dimitrovgrad, Russia, have made it, Los Alamos records.

Why it matters

Californium-252 is a very efficient neutron source, Los Alamos says. It halves in 2.647 years for IAEA, 2.645 for the Atomic Mass Data Center and 2.65 for Los Alamos. IAEA gives 96.898 per cent of its decays to alpha emission and 3.102 per cent to spontaneous fission. Its neutron energy spectrum closely resembles a fission reactor’s, and a small quantity makes an ideal portable source for work that needs only a weak stream of neutrons, IUPAC notes.

Neutron moisture gauges carry it, and well-logging uses it to locate water- and oil-bearing layers. It also serves on-site activation analysis in prospecting: neutrons strike a sample, and its emissions reveal silver or gold ore. Prompt gamma neutron activation analysis checks coal, cement and minerals for many elements at once, non-destructively. Medicine has used it in boron neutron capture therapy, delivering neutrons close to a tumour. It has also served brachytherapy, radiation from a source inside the body, for many cancers, IUPAC notes.

Californium-249 has served as a target too. Scientists at Dubna and Lawrence Livermore bombarded it with calcium-48 and made three atoms of oganesson, announced in October 2006, according to Jefferson Lab. Oak Ridge had produced and sold more than 63 mg of californium-252 by May 1975, Los Alamos records.

Properties

Properties of californium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	98
Atomic mass	251.07959 Da	Da	PubChem, longest-lived isotope
Melting point	900 °C	°C	Los Alamos: 900 °C
Boiling point	1470 °C	°C	Los Alamos: 1470 °C
Electronegativity (Pauling)	1.3
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f¹⁰
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Californium’s hazard is ionising radiation: alpha particles from its long-lived isotopes, californium-249 to californium-252, and neutrons in large numbers from californium-252. Los Alamos warns that this neutron output poses biological hazards. In Haz-Map’s radionuclide summary, californium-252 is the most important radionuclide, bone is the critical organ and internal toxicity is rated very high. The hazard record is in PubChem’s Laboratory Chemical Safety Summary for the element.

Questions

How is californium metal made?

The metal has been made by heating californium trifluoride with lithium, and by reducing the oxide with thorium or lanthanum, in work by R. G. Haire in 1982, Los Alamos records. Los Alamos gives about 10 milligrams as the largest amount prepared at one time. At room temperature the metal takes a double hexagonal close-packed structure, and at high temperature a face-centred cubic one. Small pieces left in air or moisture grow an oxide coat, though not violently.

What compounds does californium form?

Its compounds include oxides, halides, oxyhalides, hydrides, tellurides and a few organo-californium compounds, according to Los Alamos. Among them are californium trichloride, CfCl₃, and californium oxychloride, CfOCl, Jefferson Lab records. The oxychloride was the first californium compound whose structure was well defined, worked out by Cunningham and Wallmann about ten years after the element's discovery, Los Alamos records. When heated, the metal reacts with nitrogen, hydrogen or a halogen, and it reacts rapidly with aqueous mineral acids, HSDB adds.

What is the longest-lived isotope of californium?

Californium-251, with a half-life of 898 years in the IAEA's evaluation and 900 years in HSDB's. It decays by alpha emission to curium-247 and also by spontaneous fission. Two neighbours last far less: californium-249 halves in 351 years and californium-250 in 13.08 years, in the Atomic Mass Data Center's figures. No isotope of the element is stable, and PubChem gives the mass of its longest-lived isotope, 251.080 Da.

Is californium dangerous?

Yes, as a source of ionising radiation. Its long-lived isotopes emit alpha particles, and californium-252 also sends out neutrons in such numbers that Los Alamos warns of biological hazards. Haz-Map rates the internal toxicity of californium-252 very high and names bone as the organ at greatest risk. The fuller hazard record for the element sits in PubChem's Laboratory Chemical Safety Summary.