1s² 2s²p⁶ 3s²p⁶d¹⁰ 4s²p⁶d¹⁰ 5s²p⁶ 6s¹

Characteristics

The electromagnetic spectrum of Cesium has two bright lines in the blue part of the spectrum along with several other lines in the red, yellow, and green.   This metal is silvery gold in color and is both soft and ductile.  Cesium is the second most electropositive and alkaline of the chemical elements and has the second lowest ionization potential (after Francium).  Cesium is the least abundant of the five non-radioactive alkali metals.   (Technically, Francium is the least common alkali metal, but since it is highly radioactive with an estimated 550 grams in the entire Earth's crust at one time,  its abundance can be considered zero in practical terms.)

Along with Gallium, Francium, and Mercury, Cesium is among the only metals that are liquid at or near room temperature.  Cesium reacts explosively in cold water and also reacts with ice at temperatures above -116°C (157°K).

Cesium Hydroxide (CsOH) is a very strong base and will rapidly etch the surface of glass.  CsOH is often stated to be the "strongest base" (after FrOH), but in fact many compounds such as n-butyllithium and Sodium Amide are stronger.

There is an account that Cesium, reacting with Fluorine, takes up more Fluorine than it stoichiometrically should.  It is possible that, after the salt Cs⁺F^- has formed, the Cs⁺ ion, which has the same electronic structure as elemental Xenon, can like Xenon be oxidized further by Fluorine and form traces of a higher Fluoride such as CsF₃, analogous to XeF₂.

Occurrence

An alkali metal, Cesium occurs in Lepidolite, Pollucite (hydrated Silicate of Aluminum and Cesium) and within other sources. One of the world's most significant and rich sources of this metal is at Bernic Lake in Manitoba, Canada.  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 Cesium Cyanide and in a number of other ways.  Exceptionally pure and gas-free Cesium can be made by the thermal decomposition of Cesium Azide.  The primary compounds of Cesium are Cesium Chloride and its Nitrate.  The price of Cesium metal in 1997 was about $30 per gram, but its compounds are much cheaper.

Applications

Probably the most widespread use of Cesium today is in Cesium Formate-based drilling fluids for the oil industry.  The high density of the Cesium Formate brine (up to 2.3 sg,) coupled with the relative benignity of ¹³³Cs , reduces the requirement for toxic high-density suspended solids in the drilling fluid, which is a significant technological, engineering and environmental advantage.

Cesium is also 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.  SI defines the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two electron spin energy levels of the ground state of the ¹³³Cs atom.

• 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 in this function.
• Like other elements of group 1, Cesium has a great affinity for Oxygen and is used as a "getter" in vacuum tubes.
• This metal is also used in photoelectric cells due to its ready emission of electrons.
• Cesium is used as a catalyst in the hydrogenation of certain organic compounds.
• Radioactive isotopes of Cesium are used in the medical field to treat certain types of cancer. 
• Cesium Fluoride is widely used in organic chemistry as a base and as a source of Anydrous Floride Ion.
• Cesium vapor is used in many common magnetometers.
• Because of their high density, Cesium 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.
• Cesium Nitrate is used as oxidizer to burn Silicon in infrared flares like the LUU-19 flare, because it emits much of its light in the near infrared spectrum.
• More recently this metal has been used in ion propulsion systems.
• Cesium-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 (used to measure the thickness of rock-strata)
• also used as an internal standard in spectrophotometry

Compounds

Isotopes

Cesium 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 meltdown.   It's because this isotope (¹³⁷Cs) is one of the numerous products of fission, which directly issue from the fission of an Uranium core.

Beginning in 1945 with the commencement of nuclear testing, ¹³⁷Cs was released into the atmosphere where it is 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 can be estimated as a function of time.  Cesium-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

All alkali metals are highly reactive. Cesium, 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 (even cold water or ice).

  Cesium Hydroxide is an extremely strong base, and can etch glass.

Cesium compounds are encountered rarely by most persons . All Cesium 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.  Rats fed Cesium in place of Potassium in their diet 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 which varies depending on location.  Radiocesium 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.

(L. caesius, sky blue) Cesium was discovered spectroscopically by Bunsen and Kirchhoff in 1860 in mineral water from Durkheim. Cersium, an alkali metal, occurs in lepidolite, pollucte (a hydrated silicate of aluminum and cesium), and in other sources. One of the world's richest sources of cesium is located at Bernic Lake, Manitoba. The deposits are estimated to contain 300,000 tons of pollucite, averaging 20% cesium. It can be isolated by electrolysis of the fused cyanide and by a number of other methods. Very pure, gas-free cesium can be prepared by thermal decomposition of cesium azide. The metal is characterized by a spectrum containing two bright lines in the blue along with several others in the red, yellow, and green. It is silvery white, soft, and ductile. It is the most electropositive and most alkaline element. Cesium, gallium, and mercury are the only three metals that are liquid at room temperature. Cesium reacts explosively with cold water, and reacts with ice at temperatures above -116^oC. Cesium hydroxide, the strongest base known, attacks glass. Because of its great affinity for oxygen the metal is used as a "getter" in electron tubes. It is also used in photoelectric cells, as well as a catalyst in the hydrogenation of certain organic compounds. The metal has recently found application in ion propulsion systems. Cesium is used in atomic clocks, which are accurate to 5 s in 300 years. Its chief compounds are the chloride and the nitrate. Cesium has 32 isotopes (more than any element) with masses ranging from 114 to 145. The present price of cesium is about $30/g.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond

Cesium was discovered by Robert Wilhelm Bunsen and Gustav Robert Kirchoff, German chemists, in 1860 through the spectroscopic analysis of Durkheim mineral water. They named cesium after the blue lines they observed in its spectrum. Today, cesium is primarily obtained from the mineral pollucite (CsAlSi₂O₆). Obtaining pure cesium is difficult since cesium ores are frequently contaminated with rubidium, an element that is chemically similar to cesium. To obtain pure cesium, cesium and rubidium ores are crushed and heated with sodium metal to 650°C, forming an alloy that can then be separated with a process known as fractional distillation. Metallic cesium is too reactive to easily handle and is usually sold in the form of cesium azide (CsN₃). Cesium is recovered from cesium azide by heating it.

Cesium has the second lowest melting point of all metallic elements, which limits its uses. Cesium readily combines with oxygen and is used as a getter, a material that combines with and removes trace gases from vacuum tubes. Cesium is also used in atomic clocks, in photoelectric cells and as a catalyst in the hydrogenation of certain organic compounds. Since it is easily ionized and has a high mass, cesium ions may one day be used as a propellant in ion engines on spacecraft.

Cesium reacts violently with water and ice, forming cesium hydroxide (CsOH). Cesium hydroxide is the strongest base known and will attack glass. Cesium chloride (CsCl) and cesium nitrate (CsNO₃) are cesium's most common compounds and are primarily used in the production of other chemicals.