Alkali metal
Caesium (IUPAC spelling; also spelled cesium in American English) is a chemical element; it has 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; 301.6 K), 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 with water even at −116 °C (−177 °F). It is the least electronegative stable element, with a value of 0.79 on the Pauling scale. It has only one stable isotope, caesium-133. Caesium is mined mostly from pollucite. Caesium-137, a fission product, is extracted from waste produced by nuclear reactors. It has the largest atomic radius of all elements whose radii have been measured or calculated, at about 260 picometres.
The German chemist Robert Bunsen and physicist Gustav Kirchhoff discovered caesium in 1860 by the then newly developed method of flame spectroscopy. The first small-scale applications for caesium were as a "getter" in vacuum tubes and in the light-sensitive anodes of photoelectric cells. Caesium is widely used in highly accurate atomic clocks. In 1967, the International System of Units began using a specific hyperfine transition of neutral caesium-133 atoms to define the basic unit of time, the second. …
In 1860, Robert Bunsen and Gustav Kirchhoff discovered caesium in the mineral water from Dürkheim, Germany. Because of the bright blue lines in the emission spectrum, they derived the name from the Latin word caesius, meaning 'bluish grey'. Caesium was the first element to be discovered with a spectroscope, which had been invented by Bunsen and Kirchhoff only a year previously.
To obtain a pure sample of caesium, 44,000 litres (9,700 imp gal; 12,000 US gal) of mineral water had to be evaporated to yield 240 kilograms (530 lb) of concentrated salt solution. The alkaline earth metals were precipitated either as sulfates or oxalates, leaving the alkali metal in the solution. After conversion to the nitrates and extraction with ethanol, a sodium-free mixture was obtained. From this mixture, the lithium was precipitated by ammonium carbonate. Potassium, rubidium, and caesium form insoluble salts with chloroplatinic acid, but these salts show a slight difference in solubility in hot water, and the less-soluble caesium and rubidium hexachloroplatinate ((Cs,Rb)2PtCl6) were obtained by fractional crystallization. After reduction of the hexachloroplatinate with hydrogen, caesium and rubidium were separated by the difference in solubility of their carbonates in alcohol. The process yielded 9.2 grams (0.32 oz) of rubidium chloride and 7.3 grams (0.26 oz) of caesium chloride from the initial 44,000 litres of mineral water. …
Of all elements that are solid at room temperature, caesium is the softest: it has a hardness of Mohs 0.2. It is a very ductile, pale metal, which darkens in the presence of trace amounts of oxygen. When in the presence of mineral oil (where it is best kept during transport), it loses its metallic lustre and takes on a duller, grey appearance. It has a melting point of 28.5 °C (83.3 °F), making it one of the few elemental metals that are liquid near room temperature. The others are rubidium (39 °C [102 °F]), francium (estimated at 27 °C [81 °F]), mercury (−39 °C [−38 °F]), and gallium (30 °C [86 °F]); bromine is also liquid at room temperature (melting at −7.2 °C [19.0 °F]), but it is a halogen and not a metal. Mercury is the only stable elemental metal with a known melting point lower than caesium. In addition, caesium has a rather low boiling point, 641 °C (1186 °F), the lowest of all stable metals other than mercury. Copernicium and flerovium have been predicted to have lower boiling points than mercury and caesium, but they are extremely radioactive and it is not certain that they are metals.
Caesium forms alloys with the other alkali metals, gold, and mercury (amalgams). At temperatures below 650 °C (1202 °F), it does not alloy with cobalt, iron, molybdenum, nickel, platinum, tantalum, or tungsten. It forms well-defined intermetallic compounds with antimony, gallium, indium, and thorium, which are photosensitive. …
Caesium is a relatively rare element, estimated to average 3 parts per million in the Earth's crust. It is the 45th most abundant element and 36th among the metals. Caesium is 30 times less abundant than rubidium, with which it is closely associated, chemically.
Due to its large ionic radius, caesium is one of the "incompatible elements". During magma crystallization, caesium is concentrated in the liquid phase and crystallizes last. Therefore, the largest deposits of caesium are zone pegmatite ore bodies formed by this enrichment process. Because caesium does not substitute for potassium as readily as rubidium does, the alkali evaporite minerals sylvite (KCl) and carnallite (KMgCl3·6H2O) may contain only 0.002% caesium. Consequently, caesium is found in few minerals. Percentage amounts of caesium may be found in beryl (Be3Al2(SiO3)6) and avogadrite ((K,Cs)BF4), up to 15 wt% Cs2O in the closely related mineral pezzottaite (Cs(Be2Li)Al2Si6O18), up to 8.4 wt% Cs2O in the rare mineral londonite ((Cs,K)Al4Be4(B,Be)12O28), and less in the more widespread rhodizite. The only economically important ore for caesium is pollucite Cs(AlSi2O6), which is found in a few places around the world in zoned pegmatites, associated with the more commercially important lithium minerals, lepidolite and petalite. Within the pegmatites, the large grain size and the strong separation of the minerals results in high-grade ore for mining. …
The largest present-day use of nonradioactive caesium is in caesium formate drilling fluids for the extractive oil industry. Aqueous solutions of caesium formate (HCOO−Cs+)—made by reacting caesium hydroxide with formic acid—were developed in the mid-1990s for use as oil well drilling and completion fluids. The function of a drilling fluid is to lubricate drill bits, to bring rock cuttings to the surface, and to maintain pressure on the formation during drilling of the well. Completion fluids assist the emplacement of control hardware after drilling but prior to production by maintaining the pressure.
The high density of the caesium formate brine (up to 2.3 g/cm3, or 19.2 pounds per gallon), coupled with the relatively benign nature of most caesium compounds, reduces the requirement for toxic high-density suspended solids in the drilling fluid—a significant technological, engineering and environmental advantage. Unlike the components of many other heavy liquids, caesium formate is relatively environment-friendly. Caesium formate brine can be blended with potassium and sodium formates to decrease the density of the fluids to that of water (1.0 g/cm3, or 8.3 pounds per gallon). Furthermore, it is biodegradable and may be recycled, which is important in view of its high cost (about $4,000 per barrel in 2001). …
Caesium has 41 known isotopes, ranging in mass number from 112 to 152. The only stable caesium isotope is 133Cs, with 78 neutrons.
The radioactive 135Cs has a very long half-life of about 1.33 million years, the longest of all radioactive isotopes of caesium. 137Cs and 134Cs have half-lives of 30.04 and 2.065 years, respectively. The isotopes with mass numbers of 129, 131, 132 and 136, have half-lives between a day and two weeks, while most of the other isotopes have half-lives from a few seconds to fractions of a second. At least 21 metastable nuclear isomers exist. Other than 134mCs (with a half-life of just under 3 hours), all are very unstable and decay with half-lives of a few minutes or less.
The isotope 135Cs is one of the long-lived fission products of uranium produced in nuclear reactors. However, this fission product yield is reduced in most reactors because the predecessor, 135Xe, is a potent neutron poison and frequently transmutes to stable 136Xe before it can decay to 135Cs.
The beta decay from 137Cs to 137mBa results in gamma radiation as the 137mBa relaxes to ground state 137Ba, with the emitted photons having an energy of 0.6617 MeV. 137Cs and 90Sr are the principal medium-lived products of nuclear fission, and the prime sources of radioactivity from spent nuclear fuel from several years to several hundred years after removal. Those two isotopes are the largest source of residual radioactivity in the area of the Chernobyl disaster. …
Nonradioactive caesium compounds are only mildly toxic, and nonradioactive caesium is not a significant environmental hazard. Because biochemical processes can confuse and substitute caesium with potassium, excess caesium can lead to hypokalemia, arrhythmia, and acute cardiac arrest, but such amounts would not ordinarily be encountered in natural sources.
The median lethal dose (LD50) for caesium chloride in mice is 2.3 g/kg, which is comparable to the LD50 values of potassium chloride and sodium chloride. The principal use of nonradioactive caesium is as caesium formate in petroleum drilling fluids because it is much less toxic than alternatives, though it is more costly.
Elemental caesium is one of the most reactive elements and is highly explosive in the presence of water. The hydrogen gas produced by the reaction is heated by the thermal energy released at the same time, causing ignition and a violent explosion. This can occur with other alkali metals, but caesium is so potent that this explosive reaction can be triggered even by cold water.
It is highly pyrophoric: the autoignition temperature of caesium is −116 °C (−177 °F), and it ignites explosively in air to form caesium hydroxide and various oxides. Caesium hydroxide is a very strong base, and will rapidly corrode glass.
The isotopes 134 and 137 are present in the biosphere in small amounts from human activities, differing by location. …