Californium

1 · Identity

Symbol, number

Cf, 98

Oxidation states in water

+3 (book entry); never measured in natural water

Note

Identity from the book entry; nothing measured in water.

Sources

Identity

Name and symbol

Californium, Cf

Atomic number

98 protons

Position

no group (f-block) · period 7 · f-block · actinide

CAS number

7440-71-3

Atomic structure

Atomic mass

251 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f¹⁰
[Rn] 7s²⁵f¹⁰

Electrons per shell

2, 8, 18, 32, 28, 8, 2

Valence electrons

12 ns, (n-1)d and (n-2)f

Physical properties

State at room temperature

Solid

Melting point

1,173 K (899.85 °C)

Boiling point

1,470 K (1,196.85 °C)

Density

not in sources

Appearance

silvery

Thermal conductivity

not in sources

Electrical resistivity

not in sources

Electrical conductivity

not in sources

Crystal structure

double hexagonal close packed

Molar heat capacity

not in sources

Chemical properties

Oxidation states

+3

Electronegativity

1.3 (Pauling Scale)

Ionisation energy

6.3 eV
1st 608 kJ/mol

Electron affinity

not in sources

Atomic radius

van der Waals 245 pm

Ionic radius

Cf³⁺ 95; Cf⁴⁺ 82 pm

Reactivity

A fairly reactive silvery actinide of the second half of the series, whose shielded 5f electrons make it behave like the lanthanide dysprosium: +3 is the only state stable in water, +4 (a strong oxidant) and +2 (a strong reductant) exist in solids, and a +5 state is doubtful.

with water

Tarnishes faster in moist than in dry air, small pieces or foils forming an oxide without a violent reaction; in water only the Cf3+ cation is stable and attempts to oxidize or reduce it in solution have failed.

with oxygen, air

Slowly tarnishes in air at room temperature and reacts with oxygen when heated; the oxides are yellow-green Cf2O3 and black-brown CfO2.

with acids

Dissolves rapidly in aqueous mineral acids to Cf3+, giving a soluble chloride, nitrate, perchlorate and sulfate; fluoride, oxalate and hydroxide precipitate it.

with halogens

Trihalides are known for all four halogens, CfCl3 (emerald green) made from Cf2O3 and hydrogen chloride at 500 C; CfF4 is the tetravalent fluoride and the dihalides CfBr2 and CfI2 come from reducing the trihalides, CfBr3 being reducible by hydrogen where BkBr3 is not.

Typical compounds

• Cf₂O₃ californium(III) oxide yellow-green sesquioxide, from the sulfate via hydrogen
• CfO₂ californium(IV) oxide black-brown, cubic
• CfF₃ californium(III) fluoride yellow-green, slightly soluble, precipitates Cf3+
• CfCl₃ californium(III) chloride emerald green, feedstock for the iodides
• CfI₂ californium(II) iodide deep purple, from reduction of CfI3
• CfOCl californium oxychloride the first californium compound identified

Occurrence, production and use

Crustal abundance

Not Applicable

Oceanic abundance

Not Applicable

Occurrence and sources

• synthetic only milligram amounts from neutron bombardment of plutonium-239

Extraction, production

Neutron bombardment of plutonium-239; first synthesis by 242Cm (4He, n) 245Cf

nuclear reaction as printed by PubChem

Uses

Californium-252, an isotope with a half-life of about 2.6 years, is a very strong neutron source. One microgram (0.000001 grams) of californium-252 produces 170,000,000 neutrons per minute. It is being used as a neutron source to identify gold and silver ores through a technique known as neutron activation. It is also being used in devices known as neutron moisture gauges that are used to find water and oil bearing layers in oil wells.

A few compounds of californium have been produced and studied. They include: californium oxide (CfO3), californium trichloride (CfCl3) and californium oxychloride (CfOCl).

Californium's most stable isotope, californium-251, has a half-life of about 898 years. It decays into curium-247 through alpha decay or decays through spontaneous fission.

Safety, toxicity

not in sources

Discovery and name

Discovered by

S. G. Thompson, K. Street, Jr., A. Ghiorso and G. T. Seaborg (Lawrence Berkeley National Laboratory)

Discovered

1950

First isolated

R. G. Haire and R. D. Baybarz

Named by

not in sources

Origin of the name

after California, where it was discovered

Californium does not occur naturally in the Earth’s crust. It was first synthesized in 1950 by Glenn T. Seaborg and his team at the University of California using the reaction 242Cm (4He, n) 245Cf. The element was named for the state where it was first synthesized.

Californium is the second half of the actinide series where its f electrons are further removed or shielded from the valence electrons that those of the lighter actinides. Thus californium resembles the behavior of the lanthanide elements exhibiting divalent, trivalent, and tetravalent oxidation states in solid-state compounds. In solution, the trivalent state is the most stable however the divalent, tetravalent and a possible pentavalent state have been reported. The existence of Cf(V) is questionable.

Californium metal is fairly reactive. On standing in air or moisture, small pieces or foils of Cf metal quickly form an oxide but not in a violent reaction. Two methods have been successful for preparation of Cf metal: reduction of californium trifluoride with lithium metal at elevated temperature and using thorium or lanthanum metal to reduce californium oxide (R. G. Haire, 1982). The largest amount of metal prepared at one time was about 10 milligrams. The metal was eventually determined to be trivalent with a room-temperature double hexagonal close-packed structure. A face centered cubic structure has also been observed for californium metal at high temperature.

Some alloys and numerous solid-state compounds have been prepared with californium in spite of the fact that only small amounts of the element are available at any one time. Californium compounds include oxides, halides, oxyhalides, pnictides, chacogenides hydrides, tellurides, oxysulfate and oxysulfide to name a few. Some organo-californium coumpounds have also been prepared.

Because californium is a very efficient source of neutrons, many new uses are expected for it. It has already found use in neutron moisture gauges and in well-logging (the determination of water and oil-bearing layers). It is also being used as a portable neutron source for discovery of metals such as gold or silver by on-the-spot activation analysis. 252Cf is now being offered for sale by the Oak Ridge National Laboratory at a cost of $10/mg. As of May, 1975, more than 63 mg have been produced and sold. It has been suggested that californium may be produced in certain stellar explosions, called supernovae, for the radioactive decay of 254Cf (55-day half-life) agrees with the characteristics of the light curves of such explosions observed through telescopes. This suggestion, however, is questioned.

Further reading: Richard G. Haire (2006) Chapter 11, "The Chemistry of the Actinide and Transactinide Element," Third Edition, L. R. Morss, J. Fuger, and N. M. Edelstein, Eds, Springer Publishers.

This element reviewed and Updated by Dr. David Hobart, 2011