Californium (98)

Californium is a radioactive rare earth metal named after the state of California and the University of California (USA). Californium-252 is a strong neutron emitter and one microgram emits 170 million neutrons per minute, making it a biological hazard. It has a few specialized uses but only a few of its compounds are known.

Name: Californium Symbol: Cf Atomic number: 98 Atomic weight: [ 251 ] Standard state: solid at 298 K CAS Registry ID: 7440-71-3

Group in periodic table: Group name: Actinoid Period in periodic table: 7 (actinoid) Block in periodic table: f-block Color: unknown, but probably metallic and silvery white or grey in appearance Classification: Metallic

Historical information

Californium was discovered by Glenn T. Seaborg, Stanley G. Thompson, Albert Ghiorso, and Kenneth Street at 1950 in USA. The element was named after the State and University of "California.”.

The element was first made at the University of California, Berkeley, in 1950 by bombarding curium with alpha particles (helium-4 ions). It is an actinide element, the sixth trans-uranium element to be synthesized, and has the second-highest atomic mass of all the elements that have been produced in amounts large enough to see with the unaided eye (after einsteinium). The element was named after California and the University of California. It is the heaviest element to occur naturally on Earth; heavier elements can only be produced by synthesis.

Weighable quantities of californium were first produced by the irradiation of plutonium targets at the Materials Testing Reactor at the Idaho National Laboratory; and these findings were reported in 1954. The high spontaneous fission rate of californium-252 was observed in these samples. The first experiment with californium in concentrated form occurred in 1958. The isotopes californium-249 to californium-252 were isolated that same year from a sample of plutonium-239 that had been irradiated with neutrons in a nuclear reactor for five years. Two years later, in 1960, Burris Cunningham and James Wallman of the Lawrence Radiation Laboratory of the University of California created the first californium compounds—californium trichloride, californium oxychloride, and californium oxide—by treating californium with steam and hydrochloric acid.

The High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, started producing small batches of californium in the 1960s. By 1995, the HFIR nominally produced 500 milligrams of californium annually. Plutonium supplied by the United Kingdom to the United States under the 1958 US-UK Mutual Defense Agreement was used for californium production.

The Atomic Energy Commission sold californium-252 to industrial and academic customers in the early 1970s for $10 per microgram and an average of 150 mg of californium-252 were shipped each year from 1970 to 1990. Californium metal was first prepared in 1974 by Richard G. Haire and Russell D. Baybarz who reduced californium(III) oxide with lanthanum metal to obtain microgram amounts of sub-micrometer thick films

Physical properties

• Melting point: 1173 [or 900 °C (1652 °F)] K
• Boiling point: 1745 K
• Density of solid: 15100 kg m^-3

·         Below 51 K (−220 °C) californium metal is either ferromagnetic or ferrimagnetic (it acts like a magnet)

·         Between 48 and 66 K it is antiferromagnetic (an intermediate state)

·         Above 160 K (−110 °C) it is paramagnetic (external magnetic fields can make it magnetic).

Orbital properties

• Ground state electron configuration:  [Rn].5f¹⁰.7s²
• Shell structure:  2.8.18.32.28.8.2
• Term symbol:   ⁵I₈
• Pauling electronegativity: 1.3 (Pauling units)
• First ionization energy: 608 kJ mol^-1
• Second ionization energy: no data kJ mol^-1

Uses

·         Californium-252 (half-life of 2.645 years) is produced in nuclear reactors and has found a variety of uses.

·         It is used as a neutron emitter, providing neutrons for the start-up of nuclear reactors.

·         It has also been used as a target material for producing transcalifornium elements. Ununoctium, the heaviest of the elements, was produced when a californium target was bombarded with calcium ions.

·         Californium-252 is used in to treat cervical cancer. It is also used to analyze the sulfur content of petroleum and in neutron moisture gauges to measure the moisture content of soil.

Abundance and Isotopes

·         Abundance in the earth’s crust: none

·         Abundance in the solar system: negligible

·         Source: Californium is a synthetic element and is not found naturally on Earth. The spectrum of californium-254 has been observed in supernovae. Californium is produced in nuclear reactors by bombarding plutonium with neutrons and in particle accelerators.

·         Isotopes: Californium has 20 isotopes whose half-lives are known, with mass numbers 237 to 256. Californium has no naturally occurring isotopes. Its longest lived isotopes are ²⁵¹Cf, with a half-life of 898 years, ²⁴⁹Cf with a half-life of 351 years and ²⁵⁰Cf with a half-life of 13.08 years