Цезий

Описание

Cesium was discovered in 1860 by Robert Bunsen and Gustav Kirchoff. It is used in the most accurate atomic clocks. Cesium melts at 28.41°C (just below body temperature) and occurs in Earth’s crust at 2.6 ppm. Cesium is the rarest of the naturally occurring alkali metals as the isotope ¹³³Cs. Its compounds are correspondingly rare. Granites contain about 1 ppm cesium and sedimentary rocks contain approximately 4 ppm cesium. The most common commercial source of cesium is pollucite, which contains between 5 and 32% cesium oxide. Radioactive forms of cesium (¹³⁴Cs and ¹³⁷Cs) can also be found in the environment. They are produced during nuclear fission, and are used in cancer treatment.

Химические свойства

Cesium is silvery gold, soft ductile metal. It is the most electropositive and alkaline element. Cesium, gallium, and mercury are the only three metals that are liquid at or around room temperature. Cesium reacts explosively with cold water, and reacts with ice at temperatures above -116℃. Cesium hydroxide is a strong base and attacks glass and reacts with halogens to form a fluoride, chloride, bromide, and iodide. Cesium metal oxidizes rapidly when exposed to air and can form the dangerous superoxide on its surface. Most cesium compounds are water soluble.

Изотопы

Cs-133 is the only stable isotope of cesium, and it makes up all of the naturallyoccurring cesium found in the Earth’s crust. In addition to Cs-133 there are about 36radioactive isotopes of Cs, most of which are artificially formed in nuclear reactors. Allare produced in small numbers of atoms with relatively short half-lives. The range of Csisotopes is from Cs-113 (amu = 112.94451) to Cs-148 (amu = 147.94900). Most ofthese radioisotopes produce beta radiation as they rapidly decay, with the exception ofCs-135, which has a half-life of 3×10⁶yr, which makes it a useful research tool. Cs-137,with a half-life of 33 years, produces both beta and gamma radiation.

Происхождение имени

In 1860 Gustav Kirchhoff and Robert Bunsen named the element “Cesium,” using the Latin word caesius, which means bluish-gray.

Вхождение

The stable form of Cs-133 is the 48th most abundant element on Earth, but because it isso reactive, it is always in compound form. The Earth’s crust contains only about 7 ppm ofCs-133. Like the other alkali metals, it is found in mixtures of complex minerals. Its mainsource is the mineral pollucite (CsAlSi₂O₆). It is also found in lepidolite, a potassium ore.Pollucite is found in Maine, South Dakota, Manitoba, and Elba and primarily in Rhodesia,South Africa.One problem in refining cesium is that it is usually found along with rubidium; therefore,the two elements must be separated after they are extracted from their sources. The mainprocess to produce cesium is to finely grind its ores and then heat the mix to about 600°Calong with liquid sodium, which produces an alloy of Na, Cs, and Ru, which are separatedby fractional distillation. Cesium can also be produced by the thermochemical reduction of amixture of cesium chloride (CsCl) and calcium (Cs).

Характеристики

Cesium is located between rubidium and francium in group 1 of the periodic table. It isthe heaviest of the stable alkali metals and has the lowest melting point. It is also the mostreactive of the alkali metals.Cesium will decompose water, producing hydrogen, which will burn as it is liberated fromH₂O. Cesium is extremely dangerous to handle and will burn spontaneously or explode whenexposed to air, water, and many organic compounds.

Использование

Cesium is used in photovoltaic cells, vacuum tubes, scintillation counters, and atomic clocks.

Подготовка

Although Cesium metals have been prepared by fused salt electrolysis, the highly reactive nature of the metals complicates the collection step and favors the use of other preparative methods where the metals can be removed in vapor form from the reaction mixture. The oxides, hydroxides, carbonates, halides, sulphates, chromates and nitrates of Cesium have been reduced to the metals by strong reducing metals such as sodium, calcium, magnesium, barium, iron, zirconium, aluminum or silicon at moderately high temperatures. The preferred method, however, involves the reduction of the anhydrous metal chlorides with calcium metal under vacuum. Anhydrous cesium chloride is mixed with a large excess of calcium chips and heated under vacuum at 700-800°C. As the chloride is reduced, metal vapors issue from the reaction mixture and are led under the vacuum to a cooler portion of the vessel where they condense and drop into a collection vessel.

Определение

A soft golden highly reactive low-melting element of the alkali-metal group. It is found in several silicate minerals, including pollucite (CsAlSi₂O₆). The metal oxidizes in air and reacts violently with water. Cesium is used in photocells, as a catalyst, and in the cesium atomic clock. The radioactive isotopes ¹³⁴Cs (half life 2.065 years) and ¹³⁷Cs (half life 30.3 years) are produced in nuclear reactors and are potentially dangerous atmospheric pollutants.

Общее описание

A soft metallic solid. Melts at 85°F. Causes burns to skin and eyes.

Реакции воздуха и воды

Highly flammable. Cesium is spontaneously flammable in air at room temperature, if the surface is clean [Merck 11th ed. 1989]. Reacts with water to generate enough heat to ignite the hydrogen produced during the reaction, and to generate caustic Cesium hydroxide [Mellor 2 419 1946-47].

Профиль реактивности

Cesium METAL reacts violently with oxidizing agents, even weaker ones. Reacts with boron trifluoride with incandescence when heated [Merck 11th ed. 1989]. Reacts explosively with maleic anhydride [Chem Safety Data Sheet SD-88 1962; Chem. Haz. Info. Series C-71 1960]. Burns in chlorine with a luminous flame [Mellor 2 Supp. 1:380 1956]. Reacts violently with most acids. Reacts violently with fluorine, chlorine, bromine and iodine. Reacts with incandescence with sulfur and phosphorus. Burns vigorously in air.

Опасность

Although cesium has many of the properties and characteristics of the other alkali metals,because of the large size of its atoms, cesium metal is much more reactive and dangerousto handle. Special precautions need to be taken to keep it away from air, water, and organicsubstances with which it can vigorously react. Its use should be restricted to laboratories andindustries capable of using it safely.
Cesium-137, with a half-life of about 30 years, produces dangerous radiation and can causeradiation poisoning if mishandled. It is used to sterilize wheat, potatoes, and other foods toprotect them from insect damage and rotting. It is also used to kill bacteria in the treatmentof sewage sludge.

Угроза здоровью

Inhalation or contact with vapors, substance or decomposition products may cause severe injury or death. May produce corrosive solutions on contact with water. Fire will produce irritating, corrosive and/or toxic gases. Runoff from fire control may cause pollution.

Пожароопасность

Produce flammable gases on contact with water. May ignite on contact with water or moist air. Some react vigorously or explosively on contact with water. May be ignited by heat, sparks or flames. May re-ignite after fire is extinguished. Some are transported in highly flammable liquids. Runoff may create fire or explosion hazard.

Профиль безопасности

Moderately toxic by intraperitoneal route. Cesium is quite similar to potassium in its elemental state. It has been shown, however, to have pronounced physiological action in experimentation with animals. Hyper-irritability, including marked spasms, has been shown to follow the administration of cesium in amounts equal to the potassium content of the diet. It has been found that replacing the potassium in the diet of rats with cesium caused death after 10-17 days. Ignites spontaneously in air. Violent reaction with water, moisture, or steam releases hydrogen gas whch explodes. Violent reaction with acids, halogens, and other oxidizing materials. Incandescent reaction with nonmetals (e.g., sulfur, phosphorus). See also SODIUM.

Экологическая судьба

Stable cesium was shown to affect various central nervous system functions, mainly involving displacing potassium, with which it competes for transport through the potassium channel, and it can also activate sodium pump and subsequent transport into the cell across membranes. Thus, this resulted in potassium deficiency.
Radioactive isotopes of cesium, such as ¹³⁴Cs and ¹³⁷Cs, are a greater health concern than stable cesium. These radioactive isotopes of cesium are formed during nuclear fission. Both ¹³⁴Cs and ¹³⁷Cs emit beta and gamma radiations. Beta radiation travels short distances and can penetrate the skin and superficial body tissues, whereas gamma radiation can travel great distances and penetrate the entire body. Both beta and gamma radiations may induce tissue damage and disruption of cellular function.