Caesium
Caesium 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, 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. 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 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.
Since the 1990s, the largest [|application of the element] has been as caesium formate for drilling fluids, but it has a range of applications in the production of electricity, in electronics, and in chemistry. The radioactive isotope caesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology.
Spelling
Caesium is the spelling recommended by the International Union of Pure and Applied Chemistry. The American Chemical Society has used the spelling cesium since 1921, following Webster's New International Dictionary. The element was named after the Latin word caesius, meaning "bluish grey". In medieval and early modern writings caesius was spelled with the ligature æ as cæsius; hence, an alternative but now old-fashioned orthography is cæsium.Characteristics
Physical properties
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, it loses its metallic lustre and takes on a duller, grey appearance. It has a melting point of, making it one of the few elemental metals that are liquid near room temperature. The others are rubidium, francium, mercury, and gallium ; bromine is also liquid at room temperature, 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,, 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. At temperatures below, 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. It mixes with all the other alkali metals ; the alloy with a molar distribution of 41% caesium, 47% potassium, and 12% sodium has the lowest melting point of any known metal alloy, at. A few amalgams have been studied: is black with a purple metallic lustre, while CsHg is golden-coloured, also with a metallic lustre.
The golden colour of caesium comes from the decreasing frequency of light required to excite electrons of the alkali metals as the group is descended. For lithium through rubidium this frequency is in the ultraviolet, but for caesium it enters the blue–violet end of the spectrum; in other words, the plasmonic frequency of the alkali metals becomes lower from lithium to caesium. Thus caesium transmits and partially absorbs violet light preferentially while other colours are reflected; hence it appears yellowish. Its compounds burn with a blue or violet colour.
Allotropes
Caesium exists in the form of different allotropes; one of them is a dimer, called dicaesium.Chemical properties
Caesium metal is highly reactive and pyrophoric. It ignites spontaneously in air, and reacts explosively with water even at low temperatures, more so than the other alkali metals. It reacts with ice at temperatures as low as. Because of this high reactivity, caesium metal is classified as a hazardous material. It is stored and shipped in dry, saturated hydrocarbons such as mineral oil. It can be handled only under inert gas, such as argon. However, a caesium-water explosion is often less powerful than a sodium-water explosion with a similar amount of sodium. This is because caesium explodes instantly upon contact with water, leaving little time for hydrogen to accumulate. Caesium can be stored in vacuum-sealed borosilicate glass ampoules. In quantities of more than about, caesium is shipped in hermetically sealed, stainless steel containers.The chemistry of caesium is similar to that of other alkali metals, in particular rubidium, the element above caesium in the periodic table. As expected for an alkali metal, the only common oxidation state is +1. It differs from this value in caesides, which contain the anion and thus have caesium in the −1 oxidation state. Under conditions of extreme pressure, theoretical studies indicate that the inner 5p electrons could form chemical bonds, where caesium would behave as the seventh 5p element, suggesting that higher caesium fluorides with caesium in oxidation states from +2 to +6 could exist under such conditions. Some slight differences arise from the fact that it has a higher atomic mass and is more electropositive than other alkali metals. Caesium is the most electropositive chemical element. The caesium ion is also larger and less "hard" than those of the lighter alkali metals.
Compounds
Most caesium compounds contain the element as the cation, which binds ionically to a wide variety of anions. One noteworthy exception is the caeside anion, and others are the several suboxides. More recently, caesium is predicted to behave as a p-block element and capable of forming higher fluorides with higher oxidation states under high pressure. This prediction needs to be validated by further experiments.Salts of are usually colourless unless the anion itself is coloured. Many of the simple salts are hygroscopic, but less so than the corresponding salts of lighter alkali metals. The phosphate, acetate, carbonate, halides, oxide, nitrate, and sulfate salts are water-soluble. Its double salts are often less soluble, and the low solubility of caesium aluminium sulfate is exploited in refining Cs from ores. The double salts with antimony, bismuth, cadmium, copper, iron, and lead are also poorly soluble.
Caesium hydroxide is hygroscopic and strongly basic. It rapidly etches the surface of semiconductors such as silicon. CsOH has been previously regarded by chemists as the "strongest base", reflecting the relatively weak attraction between the large ion and ; it is indeed the strongest Arrhenius base; however, a number of compounds such as n-butyllithium, sodium amide, sodium hydride, caesium hydride, etc., which cannot be dissolved in water as reacting violently with it but rather only used in some anhydrous polar aprotic solvents, are far more basic on the basis of the Brønsted–Lowry acid–base theory.
A stoichiometric mixture of caesium and gold will react to form yellow caesium auride upon heating. The auride anion here behaves as a pseudohalogen. The compound reacts violently with water, yielding caesium hydroxide, metallic gold, and hydrogen gas; it dissolves in liquid ammonia and then can be reacted with a caesium-specific ion exchange resin to produce tetramethylammonium auride. The analogous platinum compound, red caesium platinide, contains the platinide ion that behaves as a.
Complexes
Like all metal cations, forms complexes with Lewis bases in solution. Because of its large size, usually adopts coordination numbers greater than 6, the number typical for the smaller alkali metal cations. This difference is apparent in the 8-coordination of CsCl. This high coordination number and softness are properties exploited in separating from other cations in the remediation of nuclear wastes, where must be separated from large amounts of nonradioactive.Halides
is a hygroscopic white solid that is widely used in organofluorine chemistry as a source of fluoride anions. Caesium fluoride has the halite structure, which means that the and pack in a cubic closest packed array as do and in sodium chloride. Notably, caesium and fluorine have the lowest and highest electronegativities, respectively, among all the known elements.Caesium chloride crystallizes in the simple cubic crystal system. Also called the "caesium chloride structure", this structural motif is composed of a primitive cubic lattice with a two-atom basis, each with an eightfold coordination; the chloride atoms lie upon the lattice points at the edges of the cube, while the caesium atoms lie in the holes in the centre of the cubes. This structure is shared with CsBr and CsI, and many other compounds that do not contain Cs. In contrast, most other alkaline halides have the sodium chloride structure. The CsCl structure is preferred because has an ionic radius of 174 pm and 181 pm.