Californium
Actinide · Period 7 · f-block
A synthetic element so neutron-rich it can start a nuclear reactor
- Atomic mass
- 251 u
- State at 25 °C
- Solid
- Electronegativity
- 1.3
- Melting point
- 899.9 °C
- Boiling point
- 1,469.9 °C
- Configuration
- [Rn] 5f10 7s2
Nucleus drawn at reduced count for clarity. Drag to rotate.
Californium, explained
Video lesson in production
Bhavya’s explainer for this element will play here.
Overview
Californium is a silvery, radioactive actinide metal with the configuration 5f10 7s2. It usually forms Cf3+, but it is one of the actinides in which the +2 state becomes accessible, a sign that the 5f electrons are being held more tightly as the series progresses. Its few-microgram samples show that it is soft and reactive, slowly tarnishing in air, with chemistry that resembles dysprosium above it in the lanthanide row.
Californium's importance comes from one isotope. About 3% of californium-252 nuclei, which have a half-life of 2.6 years, decay by spontaneous fission, splitting apart and releasing neutrons without any outside trigger. One microgram emits about 2.3 million neutrons per second, making Cf-252 an intense, compact neutron source. It is used to start up nuclear reactors, to scan for explosives and to analyse ores and oil-bearing rock by neutron activation. Only two reactors, in the United States and Russia, produce it.
Discovery
Stanley Thompson, Kenneth Street Jr., Albert Ghiorso and Glenn Seaborg made californium in February 1950 at Berkeley by bombarding a few micrograms of curium-242 with alpha particles. They obtained about 5,000 atoms of californium-245, with a 44-minute half-life. The element was named after the state of California and the University of California, where it was made.
- Where it’s found
- Californium does not occur in nature; it is produced in milligram amounts per year at Oak Ridge's High Flux Isotope Reactor and Russia's SM-3 reactor.
- What it’s used for
- Reactor start-up neutron sources
- Oil-well logging
- Neutron activation analysis
- Explosives detection
- Cancer brachytherapy
- Superheavy element targets
- In living things
- Californium has no biological role; absorbed californium deposits in bone, where its radiation and neutrons damage tissue.
- Safety
- An intense source of neutrons as well as alpha and gamma radiation; sources need heavy shielding and remote handling.
How it compares
Where Californium sits among all 118 elements on the trends that shape the table.
- Electronegativity1.3Pauling
#62 of 100 elements with data
- Ionization energy608kJ/mol
#63 of 104 elements with data
- Atomic radiusNo data
Not measured for this element
- Electron affinity-97.3kJ/mol
#106 of 109 elements with data
- Density15.1g/cm³
#21 of 114 elements with data
- Melting point1,170K
#55 of 110 elements with data
- Density
- 15.1 g/cm³
- Atomic radius
- —
- Covalent radius
- —
- Molar heat
- —
- Appearance
- silvery
- Discovered by
- Lawrence Berkeley National Laboratory
Inside the atom
98 electrons arranged in 7 shells: 2 · 8 · 18 · 32 · 28 · 8 · 2.
Orbital diagram
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f10 · shorthand [Rn] 5f10 7s2
- 1s2
- 2s2
- 2p6
- 3s2
- 3p6
- 4s2
- 3d10
- 4p6
- 5s2
- 4d10
- 5p6
- 6s2
- 4f14
- 5d10
- 6p6
- 7s2
- 5f10
Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Californium bonds.
Signature reactions
The chemistry Californium is known for.
Nuclear · Alpha bombardment
Discovery of californium
Curium-242 absorbs an alpha particle and emits a neutron, producing californium-245. Only a few thousand atoms were made, identified by their alpha decay after rapid chemical separation.
Conditions: 35 MeV alpha particles from a cyclotron
Nuclear · Spontaneous fission
Spontaneous fission of californium-252
A californium-252 nucleus splits on its own into two fragments plus, on average, nearly four neutrons. This is one representative split; the free neutrons are what make Cf-252 useful.
Conditions: spontaneous; about 3% of decays
Nuclear · Heavy-ion fusion
Making oganesson
Calcium-48 nuclei fused with californium-249 to form oganesson-294, the heaviest element yet made. Only a handful of atoms have been detected over months of beam time.
Conditions: calcium-48 beam on californium target
Where you’ll meet it
Stanley Gerald Thompson
Stanley Thompson led the team that discovered californium and berkelium.
Photo: Wikimedia Commons
Golden Gate Bridge
Californium is named after the state of California, home of the Golden Gate Bridge.
Photo: Wikimedia Commons
Gold prospecting
Californium-252 neutron sources can help identify gold- and silver-bearing ores by neutron activation.
Photo: Wikimedia Commons
Brachytherapy
Californium-252 has been used in brachytherapy, placing a radiation source close to a tumour.
Photo: Wikimedia Commons
Research
No research on Californium yet
When Bhavya publishes a project that studies Californium, it will be linked here.