Californium
[Rn] 5f¹⁰ 7s² · 2 · 8 · 18 · 32 · 28 · 8 · 2
The most expensive element available in weighable amounts: a microgram of californium-252 costs tens of dollars and a gram runs to some 27 million. It is the portable neutron source for places a reactor cannot go.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 98
- Atomic mass
- 251 а.е.м. ≈
- Electron configuration
- [Rn] 5f¹⁰ 7s²
- Electrons per shell
- 2 · 8 · 18 · 32 · 28 · 8 · 2
- Block
- f-block
- Group
- no data
- Period
- 7
- Electronegativity (Pauling)
- 1.3
- Electronegativity (Allen)
- 1.3
- Atomic radius
- 245 pm
- Covalent radius
- 168 pm
- Van der Waals radius
- 245 pm
- Ionisation energy 1
- 608 kJ/mol
- Electron affinity
- -97.3 kJ/mol
- Common oxidation states
- +3
- Oxidation states
- +2, +3, +4, +5
Physical properties
- State at 25 °C
- solid
- Density
- 15.1 g/cm³
- Melting point
- 1 173 K · 899.9 °C
- Boiling point
- 1 743 K · 1 469.9 °C ≈
- Speed of sound
- no data
Thermal properties
- Heat of fusion
- no data
- Heat of vaporisation
- no data
- Specific heat capacity
- no data
- Thermal conductivity
- no data
Mechanical properties
- Young's modulus
- no data
- Shear modulus
- no data
- Bulk modulus
- no data
- Poisson ratio
- no data
- Mohs hardness
- no data
- Brinell hardness
- no data
Electrical and magnetic properties
- Electrical resistivity
- no data
- Magnetic ordering
- paramagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- no data
Crystal structure
- Crystal structure
- hexagonal close-packed (hcp)
- Lattice constants
- a = 338.4 pm · c = 1 104 pm
Abundance
- In the crust
- 0 mg/kg
- In the ocean
- 0 mg/L
- In the universe
- 0 mg/kg
- In the human body
- 0 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 249Cf | — | 351.11 yr | α |
| 251Cf | — | 898.06 yr | α |
| 252Cf | — | 2.65 yr | α, СД |
Californium-252, half-life 2.6 years, undergoes spontaneous fission 3 % of the time, which is what makes it a neutron source. Californium-251 has the smallest known critical mass, about 5 kg.
Discovery
- Year of discovery
- 1950
- Discovered by
- Thompson, Street, Ghiorso and Seaborg
- Where
- Berkeley, USA
- Origin of the name
- after California and the University of California
History
Thompson, Street, Ghiorso and Seaborg synthesised californium in 1950 by bombarding curium with alpha particles. It is named after California and its university.
Where it occurs
Absent from nature. It may form in supernova shells, but there is no direct evidence.
How it is produced
In the high-flux reactors at Oak Ridge and Dimitrovgrad by prolonged irradiation of curium. World output is about half a gram a year.
Role in living things
No biological role.
Safety
One microgram of californium-252 emits 2.3 million neutrons a second. Handling requires thick hydrogen-rich shielding.
Uses
- Portable neutron sources for reactor startup and neutron well logging
- Neutron activation analysis in prospecting for gold and oil
- Explosives detection and airport screening
- Radiotherapy of tumours resistant to X-rays
Curiosities
- Californium-252 is the only practical portable neutron source that needs no power supply.
- A gram of californium costs about as much as 500 kilograms of gold.
- Californium is used to start nuclear reactors, which need an external neutron kick.
- Californium-251 has the smallest critical mass of any known nuclide.