Caesium

Caesium (IUPAC spelling; also spelled cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of 28.5 °C (83.3 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium has physical and chemical properties similar to those of rubidium and potassium. It is pyrophoric and reacts explosively with water even at low temperatures, and it is one of the most reactive and electropositive elements. The most stable isotope, caesium-133, is the only naturally occurring isotope; numerous short-lived radioisotopes have been synthesised. Caesium is mined mostly from pollucite, a zeolite mineral, while the radioisotopes, especially caesium-137, are a fission product extracted from spent nuclear fuel. The element has a wide range of applications, from atomic clocks and drilling fluids to medical radiation sources and industrial gauges.

History

In 1860, the German chemists Robert Bunsen and Gustav Kirchhoff discovered caesium in mineral water from Dürkheim, using the newly developed method of flame spectroscopy. The name derives from the Latin caesius, meaning "sky blue" or "bluish-grey", referring to the characteristic blue spectral lines that revealed its presence. Caesium was the first element to be discovered spectroscopically. In 1882, the chemist Carl Setterberg first isolated metallic caesium by electrolysis of molten caesium cyanide. Historically, the most significant use of caesium has been in research and development, primarily in chemical and electrical applications.

Characteristics

Physical properties

Caesium is an extremely soft, ductile, pale gold metal that darkens in the presence of trace amounts of oxygen. When in a mineral oil, it loses its metallic lustre and takes on a duller, grey appearance. It has a melting point of 28.5 °C, making it one of the few metals that are liquid near room temperature; gallium, francium, mercury, and rubidium share this property. It has a low boiling point of 671 °C, the lowest of all metals except mercury. Its density is 1.93 g/cm³, which is relatively high for an alkali metal but lower than that of many other metals.

Caesium forms alloys with the other alkali metals, and with gold; it amalgamates with mercury. At temperatures below 650 °C, it alloys with cobalt, iron, molybdenum, nickel, platinum, tantalum, and tungsten. It forms well-defined intermetallic compounds with antimony, gallium, indium, and thorium, which are photosensitive. The metal has a large atomic radius and only one stable crystal structure, a body-centred cubic, at standard conditions.

Chemical properties

Caesium metal is highly reactive and pyrophoric. It ignites spontaneously in air and reacts explosively with water, even at temperatures as low as −116 °C (−177 °F). Because of this high reactivity, it is classified as a hazardous material and is stored and transported in dry saturated hydrocarbons such as mineral oil. It can be handled only under inert gas, such as argon or nitrogen. The metal dissolves slowly in anhydrous ammonia, forming a deep blue solution. Caesium is the most electropositive of the stable elements and has the lowest ionisation energy. As a result, it readily donates its single valence electron and forms ionic compounds with most non-metals.

The chemistry of caesium is dominated by the +1 oxidation state. The caesium ion (Cs⁺) is large and weakly polarising, so its salts are typically very soluble, simple, and highly ionic. Some notable caesium compounds include caesium chloride, caesium carbonate, caesium nitrate, and caesium hydroxide—the strongest base known, capable of attacking glass. Caesium forms stable salts with large anions, such as chromate, permanganate, and tetraphenylborate.

Isotopes

Caesium has 40 known isotopes, ranging in mass number from 112 to 151. Caesium-133 is the only stable isotope and the only one found in nature. Natural caesium is monoisotopic. The longest-lived radioisotope is caesium-135, with a half-life of about 2.3 million years; caesium-137 is next with a half-life of 30.17 years. Both are produced as fission products in nuclear reactors. Caesium-137, which decays by beta emission to barium-137m (a short-lived gamma emitter), is a major source of radioactive contamination and a concern in spent fuel management and radiological emergencies. Other notable isotopes include caesium-134 with a half-life of 2.07 years, and caesium-131, used in brachytherapy.

Occurrence and production

Caesium is a relatively rare element in the Earth’s crust, with an average concentration of approximately 3 parts per million. It occurs in small quantities in a number of minerals, the most important of which is pollucite (Cs,Na)₂Al₂Si₄O₁₂·2H₂O, a zeolite. Other caesium-bearing minerals include lepidolite, carnallite, and avogadrite. The world’s largest known reserves of pollucite are located at the Tanco Mine at Bernic Lake in Manitoba, Canada, and in the Bikita District of Zimbabwe. In these deposits, pollucite averages about 20% caesium by weight.

Production of caesium compounds and metal involves several steps. Pollucite ore is crushed, ground, and then treated with strong acids such as hydrochloric acid, sulfuric acid, or hydrofluoric acid to break down the aluminosilicate matrix. Caesium is then precipitated as an insoluble compound (e.g., caesium alum, caesium tetraphenylborate) or separated via solvent extraction and ion exchange. Metallic caesium is produced by the reduction of caesium compounds, usually caesium chloride, with calcium or barium at high temperatures under vacuum, or by electrolysis of molten caesium cyanide. Annual worldwide production of caesium compounds is estimated at about 20,000 tonnes, while metallic caesium production is much smaller.

Applications

Atomic clocks

The most precise application of caesium is in caesium atomic clocks. The caesium standard is the basis for the definition of the second in the International System of Units (SI). A caesium clock uses the microwave spectral line emitted by the transition between the two hyperfine ground states of caesium-133 atoms as a frequency reference. These clocks are accurate to within one second over millions of years and are integral to global navigation satellite systems (GPS, GLONASS, Galileo), telecommunications, and scientific measurements.

Oil and gas exploration

Caesium formate, a heavy brine, is used as a drilling fluid and completion fluid in high-pressure, high-temperature oil and gas wells. Its high density (up to 2.3 g/cm³) helps control formation pressures and stabilise the wellbore, while its benign environmental profile and compatibility with reservoir formations make it preferable to traditional fluids like barite-weighted muds.

Medical and industrial radiation sources

Caesium-137 is used as a gamma radiation source in industrial radiography, sterilisation of medical equipment, and food irradiation. It has also been employed in cancer radiotherapy (teletherapy), although many such units have been replaced by linear accelerators and cobalt-60 sources due to safety and security concerns. Caesium-131 and caesium-137 are used in brachytherapy for treating prostate cancer and other malignancies.

Electronics and optics

Caesium is used as a getter in vacuum tubes to remove trace gases. Its low ionisation potential makes it a key component in ion propulsion systems, where caesium ions are accelerated to generate thrust for spacecraft. The metal is also employed in photomultiplier tubes, scintillation counters, and infrared detectors. Caesium compounds such as caesium iodide, caesium bromide, and caesium fluoride are used in optics and as scintillator materials.

Other applications

Caesium carbonate is used as a base in organic synthesis, especially in palladium-catalysed cross-coupling reactions. Caesium chloride is utilised in density gradient centrifugation for the separation of nucleic acids and viruses. Caesium nitrate is employed in signal flares and pyrotechnics because it produces a blueish-purple flame. Research continues into potential uses of caesium in high-temperature superconductors and magnetohydrodynamic energy conversion.

Biological role and health effects

Caesium has no known essential biological role. It is chemically similar to potassium and can be taken up by living organisms, replacing potassium in some biochemical processes. Stable caesium is considered slightly toxic; large doses can interfere with potassium metabolism and cause gastrointestinal distress, hypotension, and cardiac arrhythmias. The biological half-life of caesium in humans is about 1 to 4 months, depending on metabolic factors.

Radioactive isotopes of caesium, particularly caesium-137, pose significant health hazards. Once absorbed, caesium distributes uniformly throughout the body, concentrating in muscle tissue. Internal exposure can lead to increased cancer risk, radiation sickness, and, at high doses, death. Environmental releases of caesium-137, such as from the Chernobyl disaster in 1986 and the Fukushima Daiichi nuclear accident in 2011, have caused widespread contamination and long-term exclusion zones. Prussian blue (ferric hexacyanoferrate) is used medically to accelerate the excretion of caesium from the body.

Precautions

Metallic caesium is one of the most dangerous elements to handle. It must be stored and manipulated under an inert atmosphere to prevent violent reactions with air or moisture. The high reactivity and pyrophoric nature mean that even small spills can ignite spontaneously and cause severe burns. Caesium hydroxide, formed by reaction with water, is extremely corrosive. Radioactive caesium sources require strict nuclear regulatory control, secure storage, and shielding to protect against radiation exposure. Because of its potential use in radiological dispersal devices (“dirty bombs”), caesium-137 sources are subject to enhanced security measures worldwide.