98
  Cf  
251.000000
Californium

Name: Californium
Symbol: Cf
Atomic Number: 98
Atomic Weight: 251.000000
Family: Rare Earth Elements
CAS RN: 7440-71-3
Description: A radioactive rare earth metal.
State (25C): Solid
Oxidation states: +3

Molar Volume: unknown
Valence Electrons: 5f107s2

Boiling Point: unknown
Melting Point: 1173.2oK, 900oC, or 1652oF

Electron Energy Level: 2, 8, 18, 32, 28, 8, 2
Isotopes: 20 + None Stable
Heat of Vaporization: unknown
Heat of Fusion: unknown
Density: 15.1 g/cm3
Specific Heat: unknown
Atomic Radius: ~186 pm
Ionic Radius: 0.934
Electronegativity: 1.3 (Pauling), 1.2 (Allrod Rochow)
89
Ac
227.0
90
Th
232.0
91
Pa
231.0
92
U
238.0
93
Np
237.0
94
Pu
(244)
95
Am
(243)
96
Cm
(247)
97
Bk
(247)
98
Cf
(251)
99
Es
(252)
100
Fm
(257)
101
Md
(258)
102
No
(259)
103
Lr
(260)

1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f10 6s2p6 7s2

History

Californium was first synthesized by University of California, Berkeley researchers Stanley G. Thompson, Kenneth Street, Jr., Albert Ghiorso and Glenn T. Seaborg in 1950.   It was the sixth transuranium element to be discovered and the team announced their discovery on March 17, 1950.  It was named after the U.S. State of California and for the University of California system.

seaborg2.jpg (12329 bytes)

Glenn T. Seaborg

To produce element 98, the team bombarded a microgram-sized target of 242Cm with 35 MeV alpha particles in the 60-inch Berkeley cyclotron which produced atoms of 245Cf (half-life 44 minutes) and a free neutron.

Characteristics

Weighable amounts of californium make it possible to determine some of its properties using macroscopic quantities.

1s2
2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d10 5f10
6s2 6p6
7s2

252Cf (2.6 year half-life) is a very strong neutron emitter and is thus extremely radioactive and harmful (one microgram spontaneously emits 170 million neutrons per minute).  The decay of 254Cf (55-day half-life) may have been detected through telescopes in supernovae remnants. 249Cf is formed from the beta decay of 249Bk and most other californium isotopes are made by subjecting berkelium to intense neutron radiation in a nuclear reactor.

Occurrence

Although californium does not occur naturally on Earth, the element and its decay products occur elsewhere in the universe. Their electromagnetic emissions are regularly observed in the spectra of supernovae.

Applications

The element does have some specialist applications dealing with its radioactivity but otherwise is largely too difficult to produce to have widespread useful significance as a material. Some of its uses are:

In October, 2006 it was announced that on three occasions californium-249 atoms had been bombarded with calcium-48 ions to produce ununoctium (element 118),  making this the heaviest element ever synthesized.

Military

251Cf is famous for having a very small critical mass, high lethality, and short period of toxic environmental irradiation relative to radioactive elements commonly used for radiation explosive weaponry, creating speculation about possible use in pocket nukes.  This urban legend is unfounded since it would be very difficult to make a 251Cf bomb weighing less than 2 kg, and the costs of such a bomb would be prohibitive. Other weaponry uses, such as showering an area with californium, are not impossible but are seen as inhumane and are subject to inclement weather conditions and porous terrain considerations. Often cited as a consideration is the cost of producing californium in quantity, but the cost citations are usually due to extra fees that laboratory materials companies insert for sake of caution and market needs.  A government need not consider these as prohibitive.

Nuclear Fuel Cycle

Pertaining to californium's nuclear fuel cycle, it is important to make sure that the curium concentration in MOX Nuclear fuel is kept low, as neutron irradiation of curium will convert some of it to californium.  The californium will then cause the used fuel to be more difficult to handle as the californium is a strong neutron emitter (through spontaneous fission).  Hence the concentrations of curium and californium among the Minor actinides are important.

Compounds

In 1960 a few tenths of a microgram of compounds of californium were first produced and studied.  Included among these are: californium oxide (Cf2O3), californium trichloride (CfCl3) and californium oxychloride (CfOCl).  The only californium ion that is stable in aqueous solution is the californium (III) cation.

Isotopes

Nineteen radioisotopes of californium have been characterized, the most stable being 251Cf with a half-life of 898 years, 249Cf with a half-life of 351 years, and 250Cf with a half-life of 13 years.  All of the remaining radioactive isotopes have half-lives that are less than 2.7 years, and the majority of these have half-lives shorter than 20 minutes.  The isotopes of californium range in atomic weight from 237.062 amu (237Cf) to 256.093 amu (256Cf).

All californium isotopes are radioactive; the long-lived isotopes are produced from berkelium-249 or from californium-249. They are: californium-249 (360-year half-life); californium-250 (13-year half-life); californium-251 (800-year half-life); and californium-252 (2.65-year half-life). These isotopes have been used in tracer amounts for investigating the chemistry of californium (which exhibits an oxidation state of +3 in acidic aqueous solution) and for preparing microgram quantities of compounds such as the oxychloride CfOCl, the oxide Cf2O3, and the trichloride CfCl3. There is some evidence for a +2 state also. Metallic californium has not yet been prepared.

Califomium (III) is the only ion stable in aqueous solutions, all attempts to reduce or oxidize californium (III) having failed. The isotope 249Cf results from the beta decay of 249Bk while the heavier isotopes are produced by intense neutron irradiation by the reactions:

249Bk(n,y) rarrow.gif (63 bytes) 250Bk --B--> 250Cf and 249Cf(n,y) rarrow.gif (63 bytes) 250Cf followed by 250Cf(n,y) rarrow.gif (63 bytes) 251Cf(n,y) rarrow.gif (63 bytes) 252Cf

The existence of the isotopes 249Cf, 250Cf, 251Cf, and 252Cf makes it feasible to isolate californium in weighable amounts so that its properties can be investigated with macroscopic quantities. Californium-252 is a very strong neutron emitter. One microgram releases 170 million neutrons per minute, which presents biological hazards. Proper safeguards should be used in handling californium. Eighteen isotopes of californium are now recognized. 249Cf and 252Cf have half-lives of 351 years and 900 years, respectively.

Californium-252, an isotope with a half-life of about 2.6 years, is a very strong neutron emitter.  Because 3 percent of its decay occurs by spontaneous fission, it is industrially and medically important as a very intense point source of neutrons.  One microgram (0.000001 grams) of californium-252 produces 170 million neutrons per minute, which presents biological hazards. Proper safeguards should be used in handling californium.

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.

atom.gif (700 bytes)

Isotope  
Atomic Mass
 
Half-Life
237Cf 237.06207 2.1 seconds
238Cf 238.06141 21.1 ms
239Cf 239.06242 60 seconds
240Cf 240.06230 1.06 minutes
241Cf 241.06373 3.78 minutes
242Cf 242.06370 3.49 minutes
243Cf 243.06543 10.7 minutes
244Cf 244.066001 19.4 minutes
245Cf 245.068049 45.0 minutes
246Cf 246.0688053 35.7 hours
247Cf 247.071001 3.11 hours
248Cf 248.072185 333.5 days
249Cf 249.0748535 351 years
250Cf 250.0764061 13.08 years
251Cf 251.079587 900 years
252Cf 252.081626 2.645 years
253Cf 253.085133 17.81 days
254Cf 254.087323 60.5 days
255Cf 255.09105 85 minutes
256Cf 256.09344 12.3 minutes

Precautions

251Cf is famous for having a very small critical mass, high lethality, and short period of toxic environmental irradiation relative to radioactive elements.  Due to its extreme rarity in nature these concerns are exceedingly minimal.  There is currently no evidence that californium plays any biological role.

atom.gif (700 bytes)

Californium Data
 

Atomic Structure

  • Atomic Radius: ~186 pm
  • Atomic Volume:
  • Covalent Radius:
  • Cross Section (Thermal Neutron Capture) Barns: 2900
  • Crystal Structure: Hexagonal
  • Electron Configuration:
    1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f10 6s2p6 7s2
  • Electrons per Energy Level: 2, 8, 18, 32, 28, 8, 2
  • Ionic Radius: 0.934
  • Filling Orbital: 5f10
  • Number of Electrons (with no charge): 98
  • Number of Neutrons (most common/stable nuclide): 153
  • Number of Protons: 98
  • Oxidation States: 3
  • Valence Electron: 5f10 7s2

Chemical Properties

  • Electrochemical Equivalent: 3.1226 g/amp-hr
  • Electron Work Function:
  • Electronegativity: 1.3 (Pauling); 1.2 (Allrod Rochow)
  • Heat of Fusion: kJ/mol
  • Incompatibilities:
  • Ionization Potential
    • First: 6.3
  • Valence Electron Potential (-eV): 44.5

Physical Properties

  • Atomic Mass Average: 251
  • Boiling Point:
  • Coefficient of Lineal Thermal Expansion/K-1: N/A
  • Conductivity
    Electrical:
    Thermal: 0.1 W/cmK
  • Density: 15.1 g/cm3
  • Description:
    Man made radioactive metal.
  • Flammablity Class:
  • Freezing Point: see melting point
  • Heat of Vaporization: kJ/mol
  • Melting Point: 1173.2oK, 900oC, or 1652oF
  • Physical State (at 20C & 1atm): Solid
  • Specific Heat:

Regulatory / Health

  • CAS Number
    • 7440-71-3
  • NFPA 704
    • Health:
    • Fire:
    • Reactivity:
    • Special Hazard: Radioactive<
    • OSHA Permissible Exposure Limit (PEL)
      • No limits set by OSHA
    • OSHA PEL Vacated 1989
      • No limits set by OSHA
    • NIOSH Recommended Exposure Limit (REL)
      • No limits set by NIOSH
    • Levels In Humans:
      Note: this data represents naturally occuring levels of elements in the typical human, it DOES NOT represent recommended daily allowances.
      • Blood/mg dm-3: N/A
      • Bone: N/A
      • Liver: N/A 
      • Muscle: N/A 
      • Daily Dietary Intake: N/A
      • Total Mass In Avg. 70kg human: N/A

    Who / Where / When / How

    • Discoverer: G.T.Seaborg, S.G.Tompson, A.Ghiorso, K.Street Jr.
    • Discovery Location: Berkeley California United States
    • Discovery Year: 1950
    • Name Origin:
      After the state and University of California.
    • Abundance:
      • Earth's Crust: N/A
      • Seawater: N/Al
      • Atmosphere: N/A
      • Sun (Relative to H=1E12): N/A
    • Sources:
      Made by bombarding curium with helium ions. World production is only around a few grams per year.
    • Uses:
      Californium is available only in very small quanities so its uses are limited, however, it is used in nuclear research as a source of fission fragments. It is also useful as a neutron sorce for activation analysis to detect gold or silver. It can also be used in moisture gauges in oil wells.