caesium

caesium
Name	caesium
Number	55
Category	alkali metal
Appearance	pale gold
Standard atomic weight	132.90545196(6)
Electron configuration	[Xe] 6s1
Phase	solid
Melting point degC	28.44
Boiling point degC	671
Density g per cm3	1.93

caesium. It is a soft, silvery-golden alkali metal with remarkable physical and chemical properties, most notably its extreme reactivity with air and water. The element is characterized by having the lowest electronegativity of any stable element and is pivotal in the definition of the International System of Units. Discovered in the mid-19th century, its most stable natural isotope is a mononuclide, but it has numerous radioactive isotopes produced in nuclear reactions.

Properties

Caesium is an exceptionally soft metal, ductile at room temperature, and possesses a distinctive pale gold hue. It has the second-lowest melting point of all metallic elements, surpassed only by gallium, and a relatively low boiling point. Its single valence electron is very loosely bound, giving caesium the lowest ionization potential and making it highly electropositive. The metal reacts explosively with cold water and ignites spontaneously in air, forming a mixture of caesium hydroxide and caesium peroxide. In compounds, it most commonly exhibits the +1 oxidation state, forming salts like caesium chloride and caesium nitrate.

History

The element was first identified in 1860 by German chemists Robert Bunsen and Gustav Kirchhoff through the then-new technique of flame spectroscopy. They discovered it in mineral water from the Dürkheim spa, observing two bright blue lines in the spectrum that were not attributable to any known element. The name derives from the Latin *caesius*, meaning "sky blue," for the color of its spectral lines. The first pure metal was isolated the following year by Carl Setterberg via the electrolysis of molten caesium cyanide.

Occurrence and production

Caesium is relatively rare in the Earth's crust, occurring at an average abundance of about 3 parts per million. It does not form its own distinct minerals but is found as a trace component in several others. The primary commercial source is the mineral pollucite, a caesium aluminium silicate found in significant deposits at locations like Bernic Lake in Manitoba and in the Zimbabwean Bikita pegmatite field. Production involves mining the ore, followed by acid leaching and a complex series of purification steps, often involving solvent extraction or ion exchange, to yield compounds like caesium carbonate or caesium formate.

Applications

The most critical application of caesium is in the definition of time. The caesium-133 isotope is the basis for the caesium atomic clock, which defines the second in the International System of Units. These clocks are fundamental to the operation of the Global Positioning System and international timekeeping standards like Coordinated Universal Time. Caesium formate is used as a high-density drilling fluid in petroleum extraction. Radioactive caesium-137, a fission product from nuclear reactors, is employed in medical radiotherapy devices for treating cancer, in industrial radiography, and in certain types of moisture-density gauges.

Precautions

All caesium compounds require careful handling due to their chemical reactivity. The metal itself is classified as a hazardous material, posing severe risks of fire and explosion upon contact with air or water. Radioactive isotopes, particularly caesium-137 released during events like the Chernobyl disaster and the Fukushima Daiichi nuclear disaster, present significant environmental and health hazards due to their biological mobility and relatively long half-life. This isotope can contaminate ecosystems and enter the food chain, mimicking potassium in biological systems and increasing cancer risk from internal exposure.

Isotopes

Caesium has at least 40 known isotopes, with mass numbers ranging from 112 to 151. Naturally occurring caesium is essentially pure caesium-133, a stable isotope. The most significant artificial isotopes are caesium-134 and caesium-137, both produced as fission products in nuclear fission processes. Caesium-137 has a half-life of about 30.17 years and decays via beta decay to metastable barium-137m, emitting strong gamma radiation. Another notable isotope, caesium-135, is a long-lived fission product with a half-life of 2.3 million years, studied in the context of long-term nuclear waste management.

Category:Chemical elements Category:Alkali metals