Californium (Cf)
Californium (Cf) is a synthetic, highly radioactive element with the atomic number 98, categorized within the actinide series of the periodic table. Discovered in 1950 by a team at the University of California, Berkeley, Californium is produced through particle accelerators or nuclear reactors and is not found naturally in the environment. This element is notable for its ability to emit a large number of neutrons, particularly in the form of its isotope californium-252, which makes it valuable in applications like neutron activation analysis. This technique allows scientists to detect various elements in materials by bombarding samples with neutrons to create radioactive isotopes that emit gamma rays, which can then be analyzed.
Californium is a silvery-white metal that is soft and can be cut with a knife, though many of its physical properties remain unknown due to the small quantities in which it is produced and the risks associated with handling it. It has three oxidation states: +2, +3, and +4, with the +3 state being the most stable in aqueous solutions. The element's unique properties and radioactivity necessitate careful handling and protective measures to safeguard against health hazards. As an intriguing member of the actinide group, Californium highlights the intersection of chemistry, nuclear science, and safety considerations in modern research and industry.
Californium (Cf)
- Element Symbol: Cf
- Atomic Number: 98
- Atomic Mass: 251
- Group # in Periodic Table: n/a
- Group Name: Actinides
- Period in Periodic Table: 7
- Block of Periodic Table: f-block
- Discovered by: Stanley G. Thompson, Glenn T. Seaborg, Kenneth Street Jr., Albert Ghiorso (1950)
Californium is a silvery-white, highly radioactive element that is part of the actinide series in the periodic table. The actinides, which are metals with atomic numbers between 89 and 103, also include actinium, thorium, uranium, plutonium, curium, berkelium, fermium, and lawrencium. Some actinide elements, such as uranium and thorium, have isotopes that are naturally abundant within minerals in Earth’s crust. Radioactive decay of these natural isotopes produces other actinides, such as actinium. Some actinides are not found in nature. Heavier actinides must be produced synthetically. Californium is one of the synthetic actinides. This element is produced within particle accelerators or nuclear reactors. It can also be produced during the explosion of a thermonuclear bomb.
Californium is a transuranium element. This category of elements includes all those with atomic numbers greater than 92, the atomic number of uranium. Most actinides are also transuranium elements. These elements are radioactive and unstable. The transuranium actinides were discovered between 1940 and 1961.
Californium was discovered in 1950 by a team of American physicists at the University of California, Berkeley. This team included Stanley G. Thompson, Kenneth Street Jr., Albert Ghiorso, and Glenn T. Seaborg. These scientists had spent three years producing a sample of curium-242 large enough to use as a target within a cyclotron particle accelerator. After setting up this curium as a target, they bombarded it with helium nuclei. This process produced about five thousand atoms of a new element with the atomic number 98. The team named this element californium after the state and university where it was discovered. Giving the element this name broke the convention that had previously been followed, one in which the actinide element would be named in the same way as the lanthanide element directly above it in the periodic table. Dysprosium, which is directly above californium, was named for its property of being hard to obtain, not for its place of discovery.
Californium was one of many actinide elements with atomic numbers between 93 and 103 to be discovered by researchers at Berkeley. Seaborg in particular was also involved in the discovery of the actinides with atomic numbers from 94 to 102. The element seaborgium was named after him.
Much of the funding for early work on radioactive elements in the United States was linked to the Manhattan Project, which resulted in the development of the atomic bomb during World War II.
Physical Properties
Californium is a silvery-white metal that is solid in its standard state at 298 kelvins (K). It is a soft element that can be cut with a knife. Many properties of californium are unknown because it is produced in small quantities and can be dangerous to work with. Californium has a density of 15.1 grams per cubic centimeter (g/cm3) at standard state. Its melting point is 900 degrees Celsius (°C). Its boiling point is unknown. Also unknown are its specific heat, thermal conductivity, electrical conductivity, resistivity, and magnetic type.
Chemical Properties
Californium has a double hexagonal crystal structure at room temperature. At very high temperatures it has a face-centered cubic structure. This element is chemically reactive. When exposed to air, it produces an oxide layer on its surface. Californium forms compounds with many nonmetallic elements.
The electron affinity of californium is unknown. This element has four valence electrons. Its electronegativity is 1.3. Its ionization energies are 608 and 1,139 kilojoules per mole (kJ/mol). Californium has three oxidation states: +2, +3 and +4. Its trivalent (+3) oxidation state is most stable in an aqueous solution.
Californium has no stable isotopes. It has twenty known radioactive isotopes, with mass numbers ranging from 237 to 256. The first isotope discovered was californium-244, which was produced through alpha-particle bombardment of a curium-242 target. This isotope has a half-life of 44 minutes. Californium-249, which is produced through the beta decay of berkelium-249, has a half-life of 251 years. Californium-251 is the longest-lived isotope, with a half-life of 898 years. Californium-252 is the most commonly produced isotope due to its usefulness as a neutron emitter. It has a half-life of 2.65 years. Californium has an electron configuration of [Rn]5f107s2.
Applications
Californium is not found in nature. It is produced within particle accelerators, within nuclear reactors, or through the radioactive decay of berkelium-249. Californium metal can be produced through the reduction of californium trifluoride combined with lithium metal at high temperatures. It can also be produced by reducing californium oxide using thorium or lanthanum metal.
The californium-252 isotope is highly radioactive. It can undergo spontaneous fission to release massive numbers of neutrons. Nearly 140 million neutrons are emitted from one microgram of this isotope every minute. This property makes californium-252 an excellent source of neutrons for neutron activation analysis.
Neutron activation analysis is a technique for detecting the elements contained within a sample of material. First, a sample of the substance being tested is bombarded with neutrons. Energy from the neutrons causes stable isotopes within the sample to become radioactive. These newly radioactive isotopes release distinctive gamma-ray signatures that can then be analyzed using gamma spectrometry. Neutron activation analysis is useful for determining the presence of precious metals, such as gold and silver, in rocks and mineral deposits. It can also be used to detect layers containing oil or water underground, to screen baggage for the presence of explosive elements, or to detect heavy metals in sediments, products, or animal tissue.
Because californium-252 is so useful, it has been under active production. The Oak Ridge National Laboratory produces this isotope within its High Flux Isotope Reactor (HFIR). Californium-252 is easier to produce than other actinides, and it is cheaper to buy as well. At one point a one-microgram sample of californium-252 cost as little as $10.
The intense emission of neutrons from californium-252 makes it highly dangerous to human health. Producers and users of this isotope must take care to protect themselves from both neutron and alpha particle emissions.
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