cesium

xenon ← caesium → barium Rb ↑ Cs ↓ Fr 55Cs Periodic table
Appearance
silvery gold
General properties
Name, symbol, number	caesium, Cs, 55
Element category	alkali metal
Group, period, block	1, 6, s
Standard atomic weight	132.9054519(2) g·mol−1
Electron configuration	[Xe] 6s1
Electrons per shell	2, 8, 18, 18, 8, 1 (Image)
Physical properties
Phase	solid
Density (near r.t.)	1.93 g·cm−3
Liquid density at m.p.	1.843 g·cm−3
Melting point	301.59 K, 28.44 °C, 83.19 °F
Boiling point	944 K, 671 °C, 1240 °F
Critical point	1938 K, 9.4 MPa
Heat of fusion	2.09 kJ·mol−1
Heat of vaporization	63.9 kJ·mol−1
Specific heat capacity	(25 °C) 32.210 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k at T/K 418 469 534 623 750 940
Atomic properties
Oxidation states	1 (strongly basic oxide)
Electronegativity	0.79 (Pauling scale)
Ionization energies	1st: 375.7 kJ·mol−1
	2nd: 2234.3 kJ·mol−1
	3rd: 3400 kJ·mol−1
Atomic radius	265 pm
Covalent radius	244±11 pm
Van der Waals radius	343 pm
Miscellanea
Crystal structure	body-centered cubic
Magnetic ordering	paramagnetic[1]
Electrical resistivity	(20 °C) 205 Ω·m
Thermal conductivity	(300 K) 35.9 W·m−1·K−1
Thermal expansion	(25 °C) 97 µm·m−1·K−1
Young's modulus	1.7 GPa
Bulk modulus	1.6 GPa
Mohs hardness	0.2
Brinell hardness	0.14 MPa
CAS registry number	7440-46-2
Most stable isotopes
Main article: Isotopes of caesium
iso NA half-life DM DE (MeV) DP 133Cs 100% 133Cs is stable with 78 neutrons 134Cs syn 2.0648 y ε 1.229 134Xe β− 2.059 134Ba 135Cs trace 2.3×106 y β− 0.269 135Ba 137Cs trace 30.07 y β− 1.176 137Ba
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Caesium or cesium (pronounced /ˈsiːziəm/, SEE-zee-əm) is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (83 °F), which makes it one of only five metals that are liquid at or near room temperature.^[2] Caesium is most notably used in atomic clocks.

Caesium is the international spelling standardized by the IUPAC, but in the United States it is more commonly spelled as cesium.^[3]

Characteristics

An ampoule containing high purity caesium metal.

The emission spectrum of caesium has two bright lines in the blue area of the spectrum along with several other lines in the red, yellow, and green areas. This metal is silvery gold in color and is both soft and ductile. Caesium has the lowest ionization energy of the chemical elements. Caesium is the least abundant of the five non-radioactive alkali metals. (Francium is the least common alkali metal but it has no stable isotopes.^[4]).

Caesium, gallium, francium, rubidium, and mercury are the only pure metals liquid at or near room temperature. (Some sodium-potassium alloys are also liquid at room temperature.) Caesium reacts explosively in cold water and also reacts with ice at temperatures above –116 °C (–177 °F, 157 K).

Caesium hydroxide (CsOH) is a very strong base and will rapidly etch the surface of glass. CsOH is often stated to be the "strongest base", but in fact many compounds such as n-butyllithium and sodium amide are stronger but are not classic hydroxide bases and are destroyed by water.

Applications

Probably the most widespread use of caesium today is in caesium formate-based drilling fluids for the oil industry. The high density of the caesium formate brine (up to 2.3 sg), coupled with the relatively benign nature of natural caesium (which has minimal radioactivity because it is almost entirely composed of a stable istotope), reduces the requirement for toxic high-density suspended solids in the drilling fluid, which is a significant technological, engineering and environmental advantage.^[5][6]

Caesium is also used in atomic clocks, which are accurate to seconds over many thousands of years. Since 1967, the International System of Measurements has based its unit of time, the second, on the properties of caesium. The International System of Units (SI) defines the second as 9,192,631,770 cycles of the radiation, which corresponds to the transition between two hyperfine energy levels of the ground state of the ¹³³Cs atom.

Other applications

As the free element

• Like other elements of group 1, caesium has a great affinity for oxygen and is used as a "getter" in vacuum tubes.
• The metal is also used in photoelectric cells due to its ready emission of electrons.
• Caesium was used as a propellant in early ion engines. It used a method of ionization to strip the outer electron from the propellant by simple contact with tungsten. Caesium use as a propellant was discontinued when Hughes Research Laboratory conducted a study finding xenon gas as a suitable replacement.

As the salt

• Caesium is used as a catalyst in the hydrogenation of certain organic compounds.
• Caesium fluoride is widely used in organic chemistry as a base and as a source of anhydrous fluoride ion.
• Caesium vapor is used in many common magnetometers.^[7]
• Because of their high density, caesium chloride solutions are commonly used in molecular biology for density gradient ultracentrifugation, primarily for the isolation of viral particles, subcellular organelles and fractions, and nucleic acids from biological samples.^[8]
• Caesium nitrate is used as an oxidizer and pyrotechnic colorant to burn silicon in infrared flares^[9] such as the LUU-19 flare,^[10] because it emits much of its light in the near infrared spectrum.^[11]
• Caesium is also used as an internal standard in spectrophotometry.^[12]
• Caesium has been used to reduce the radar signature of exhaust plumes in the SR-71 Blackbird military aircraft.^[13]

Nuclear applications

• ¹³⁴Cs has been used in hydrology as a measure of caesium output by the nuclear power industry. This isotope is used because, while it is less prevalent than either ¹³³Cs or ¹³⁷Cs, ¹³⁴Cs can be produced solely by nuclear reactions. ¹³⁵Cs has also been used in this function.
• Caesium-137 is an extremely common radioisotope used as a gamma-emitter in industrial applications such as:
  • moisture/density gauges
  • leveling gauges
  • thickness gauges
  • well logging devices which are used to measure the electron density, which is analogous to the bulk density, of the rock formations.
• Radioactive isotopes of caesium are used in the medical field to treat certain types of cancer.

History

Caesium (Latin caesius meaning "blueish grey")^[14][15] was spectroscopically discovered by Robert Bunsen and Gustav Kirchhoff in 1860 in mineral water from Dürkheim, Germany. The residues of 44,000 liters of mineral water yielded several grams of caesium salt for further analysis. Its identification was based upon the bright blue lines in its spectrum and it was the first element discovered by spectrum analysis.^[16] The first caesium metal was produced in 1882 by electrolysis of caesium chloride by Carl Setterberg.^[17] Setterberg received his PhD from Kekule and Bunsen for this work. Historically, the most important use for caesium has been in research and development, primarily in chemical and electrical applications.

Occurrence

Pollucite, a caesium mineral.

An alkali metal, caesium occurs in lepidolite, pollucite (hydrated silicate of aluminium and caesium) and within other sources. One of the world's most significant and rich sources of this metal is the Tanco mine at Bernic Lake in Manitoba.^[18] The deposits there are estimated to contain 350,000 metric tons^[18] of pollucite ore at an average of composition of 24% caesium by weight.^[19][20]

It can be isolated by electrolysis of fused caesium cyanide and in a number of other ways. Exceptionally pure and gas-free caesium can be made by the thermal decomposition of caesium azide. The primary compounds of caesium are caesium chloride and its nitrate. The price of caesium metal in 1997 was about US$30 per gram, but its compounds are much cheaper.

Isotopes

Caesium has at least 39 known isotopes, which is more than any other element except francium. The atomic masses of these isotopes range from 112 to 151. Even though this element has a large number of isotopes, it has only one naturally occurring stable isotope, ¹³³Cs. Most of the other isotopes have half-lives from a few days to fractions of a second. The radiogenic isotope ¹³⁷Cs has been used in hydrologic studies, analogous to the use of ³H. ¹³⁷Cs is produced from the detonation of nuclear weapons and is produced in nuclear power plants, and was released to the atmosphere most notably from the 1986 Chernobyl disaster. This isotope (¹³⁷Cs) is one of the numerous products of fission, directly issued from the fission of uranium.

Beginning in 1945 with the commencement of nuclear weapons testing, ¹³⁷Cs was released into the atmosphere where it is not absorbed readily into solution and is returned to the surface of the earth as a component of radioactive fallout. Once ¹³⁷Cs enters the ground water, it is deposited on soil surfaces and removed from the landscape primarily by particle transport. As a result, the input function of these isotopes cannot be estimated as a function of time. Caesium-137 has a half-life of 30.17 years. It decomposes to barium-137m (a short-lived product of decay) then to a form of nonradioactive barium.

Precautions

High purity caesium-133 (preserved under argon)

All alkali metals are highly reactive. Caesium, being one of the heavier alkali metals, is also one of the most reactive and is highly explosive when it comes in contact with 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 (the same as all alkali metals)—but caesium is so reactive that this explosive reaction can even be triggered by cold water or ice at temperatures down to −116°C.

Caesium is highly pyrophoric and ignites spontaneously in air to form caesium hydroxide and various oxides. Caesium hydroxide is an extremely strong base, and can rapidly corrode glass.

Caesium compounds are rarely encountered by most persons. All caesium compounds should be regarded as mildly toxic because of its chemical similarity to potassium. Large amounts cause hyperirritability and spasms, but such amounts would not ordinarily be encountered in natural sources, so Cs is not a major chemical environmental pollutant.^[21]

The median lethal dose (LD₅₀) value for caesium chloride in mice was determined to be 2300 mg/kg which is comparable to the LD₅₀ values of potassium chloride and sodium chloride.^[22]

The isotopes ¹³⁴Cs and ¹³⁷Cs (present in the biosphere in small amounts as a result of radiation leaks) represent a radioactivity burden which varies depending on location. Radiocaesium does not accumulate in the body as effectively as many other fission products (such as radioiodine and radiostrontium), which are actively accumulated by the body. As with other alkali metals, radiocaesium washes out of the body relatively quickly in the sweat and urine. However, radiocaesium follows potassium and tends to accumulate in plant tissues, including fruits and vegetables.^[23][24]

See also

Wikisource has the text of the 1911 Encyclopædia Britannica article Caesium.

• Caesium-137
• Goiânia accident, a major radioactive contamination incident involving a rod of caesium chloride.
• Caesium compounds
• Dirty bomb

References

External links

Wikimedia Commons has media related to: Caesium

Look up caesium in Wiktionary, the free dictionary.

• Daniel A. Steck. "Cesium D Line Data". Los Alamos National Laboratory (technical report LA-UR-03-7943). http://george.ph.utexas.edu/~dsteck/alkalidata/cesiumnumbers.pdf. 
• WebElements.com – Caesium
• Humor site dedicated to caesium