Cesium

Vapor pressure
55	xenon ← cesium → barium
	periodic table
General
Name, Symbol, Number	cesium, Cs, 55
Chemical series	alkali metals
Group, Period, Block	1, 6, s
Appearance	silvery gold ;
Atomic mass	132.9054519(2) g/mol
Electron configuration	[Xe] 6s1
Electrons per shell	2, 8, 18, 18, 8, 1
Physical properties
Phase	solid
Density (near r.t.)	1.93 g/cm³
Liquid density at m.p.	1.843 g/cm³
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
Heat of vaporization	63.9 kJ/mol
Heat capacity	(25 °C) 32.210 J/(mol·K)
Atomic properties
Crystal structure	cubic body centered
Oxidation states	1; (strongly basic oxide)
Electronegativity	0.79 (Pauling scale)
Ionization energies	1st: 375.7 kJ/mol
	2nd: 2234.3 kJ/mol
	3rd: 3400 kJ/mol
Atomic radius	260 pm
Atomic radius (calc.)	298 pm
Covalent radius	225 pm
Miscellaneous
Magnetic ordering	no data
Electrical resistivity	(20 °C) 205 nΩ·m
Thermal conductivity	(300 K) 35.9 W/(m·K)
Thermal expansion	(25 °C) 97 µm/(m·K)
Speed of sound (thin rod)	(r.t.) 1.7 m/s
Bulk modulus	1.6 GPa
Mohs hardness	0.2
Brinell hardness	0.14 MPa
CAS registry number	7440-46-2
Notable isotopes

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Cesium (also spelled caesium, chemical symbol Cs, atomic number 55) is a member of the group of chemical elements known as alkali metals. Soft and silvery gold in color, it readily liquefies at warm temperatures.

It is most notably used in atomic clocks.

Discovery

Cesium (from the Latin word caesius, meaning "sky blue" or "heavenly blue") was discovered by Robert Bunsen and Gustav Kirchhoff in 1860, when they analyzed the spectrum of mineral water obtained from Dürkheim, Germany. They held a drop of the water in a flame and observed the spectral lines of several elements. Among these lines they observed two blue lines that did not come from any of the known elements. They concluded that the blue lines were produced by a hitherto unknown element, which they named cesium. Thus cesium became the first element discovered by spectral analysis. Bunsen isolated cesium salts from the spring water, but the metal itself was isolated in 1881 by Carl Setterberg, who worked in Bunsen's laboratory.

Occurrence

Pollucite, a cesium mineral.

Cesium occurs in several minerals, particularly lepidolite and pollucite (a hydrated silicate of aluminum and cesium). One of the world's most significant and rich sources of this metal is located at Bernic Lake in Manitoba. The deposits there are estimated to contain 300,000 metric tons of pollucite, at an average of 20% cesium.

It can be isolated by electrolysis of fused cyanide and in several other ways. Exceptionally pure, gas-free cesium can be made by decomposing cesium azide with heat. The primary compounds of cesium are its chloride and its nitrate.

Notable characteristics

As an alkali metal, cesium is part of group 1 of the periodic table, between rubidium and francium. Also, it lies in period 6, just before barium. This silvery gold metal is soft and ductile (it can be readily drawn into wires).

As noted above, the spectrum of cesium has two bright lines in the blue part of the electromagnetic spectrum. In addition, its spectrum has several other lines in the red, yellow, and green regions.

Cesium is the most electropositive and most alkaline of the stable chemical elements. Aside from francium, it has the lowest ionization potential of all the elements, which means that it readily loses its outermost electron to become an ion. (Ionization potential is the energy needed to remove an electron from the atom's outermost shell.)

Cesium is the least abundant of the five nonradioactive alkali metals. Technically, francium is the least common alkali metal, but because it is highly radioactive and its total worldwide quantity is estimated to be only 340 to 550 grams, its abundance can be considered zero in practical terms.^[1]

Along with gallium, francium, and mercury, cesium is among the few metals that are liquid at or near room temperature. Its melting point is 28.44°C.

Cesium reacts explosively when it comes in contact with water (even cold water), and also reacts with ice at temperatures above −116°C. The reaction with water produces cesium hydroxide (CsOH), an extremely strong chemical base that will rapidly etch the surface of glass. In addition cesium reacts violently with chlorine gas to produce cesium chloride (CsCl).

Isotopes

Cesium has at least 39 known isotopes which is more than any other element, except for francium. The atomic masses of these isotopes range from 112 to 151. Yet it has only one naturally occurring stable isotope: ¹³³Cs. Most of the other isotopes (except for those noted in the table) have half-lives from a few days to fractions of a second.

The isotope ¹³⁷Cs, with a half-life of 30.17 years, has been used in hydrologic studies, analogous to the use of tritium (³H). It decomposes to barium-137 (a short-lived product of decay), then to a form of nonradioactive barium.

Cesium-137 is produced during the detonation of nuclear weapons and in nuclear power plants. It was a notable product during the 1986 Chernobyl meltdown. Beginning in 1945, with the commencement of nuclear testing, ¹³⁷Cs has been released into the atmosphere, where it is absorbed by moisture and returned to the Earth's surface as a component of radioactive fallout. Once ¹³⁷Cs enters groundwater, it is deposited on soil surfaces and removed from the landscape primarily by particle transport.

Compounds of cesium

Cesium chloride

Cesium chloride is an ionic compound. It can be prepared by the reaction of cesium hydroxide or cesium carbonate with hydrochloric acid. The resulting cesium chloride salt is purified by recrystallization. The crystalline structure of this salt is composed of interlocking simple cubic lattices of anions and cations.

Cesium chloride is used in the preparation of electrically conducting glasses. Radioisotopes of cesium chloride are used in nuclear medicine, including treatment of cancer.

Applications

Cesium is most notably used in atomic clocks, which are accurate to seconds in many thousands of years. Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of cesium.
¹³⁴Cs has been used in hydrology as a measure of cesium 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 for this purpose.
Like other group 1 elements, cesium has great affinity for oxygen and is used as a "getter" in vacuum tubes.
This metal is also used in photoelectric cells because it readily emits electrons.
Cesium is used as a catalyst in the hydrogenation of certain organic compounds.
Radioactive isotopes of cesium are used in medicine to treat certain types of cancer.
Cesium fluoride is widely used in organic chemistry as a base and as a source of anhydrous fluoride ion.
Cesium vapor is used in many common magnetometers.
Because of their high density, cesium chloride solutions are commonly used in molecular biology experiments for the isolation of nucleic acids by a technique known as "density gradient ultracentrifugation."
Recently, this metal has been used in ion propulsion systems.
Cesium-137 is an extremely common isotope in industrial applications such as:
- moisture density gauges
- leveling gauges
- thickness gauges
- well-logging devices (used to measure the thickness of rock-strata)

Precautions

All alkali metals are highly reactive. As one of the heavier alkali metals, cesium is also one of the most reactive. As noted above, it is highly explosive when it comes in contact with water or ice. Cesium hydroxide is an extremely strong base and can attack glass.

All cesium compounds should be regarded as mildly toxic, because of its chemical similarity to potassium. Contact with large amounts can cause hyperirritability and spasms. On the other hand, such amounts would not ordinarily be encountered in nature, so cesium is not a major chemical environmental pollutant. Rats fed with cesium in place of potassium in their diet were found to die, so this element cannot replace potassium in function.

The isotopes ¹³⁴Cs and ¹³⁷Cs (present in the biosphere in small amounts as a result of radiation leaks) represent a radioactivity burden that varies depending on location. Radioactive cesium does not accumulate in the body as effectively as many other fission products, such as radioactive iodine or strontium.

Footnote

↑ Adloff, Jean-Pierre and George B. Kauffman (09/23 2005). Francium (Atomic Number 87), the Last Discovered Natural Element. The Chemical Educator 10 (5).

Reference

Los Alamos National Laboratory - Cesium

External links

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[1] Adloff, Jean-Pierre and George B. Kauffman (09/23 2005). Francium (Atomic Number 87), the Last Discovered Natural Element. The Chemical Educator 10 (5).

[1]