60 | The History and Use of Our Earth’s Chemical Elements
60 | The History and Use of Our Earth’s Chemical Elements
ISOTOPES:Cs-133istheonlystableisotopeofcesium,anditmakesupallofthenaturally occurringcesiumfoundintheEarth’scrust.InadditiontoCs-133thereareabout36 radioactiveisotopesofCs,mostofwhichareartificiallyformedinnuclearreactors.All areproducedinsmallnumbersofatomswithrelativelyshorthalf-lives.TherangeofCs isotopesisfromCs-113(amu=112.94451)toCs-148(amu=147.94900).Mostof theseradioisotopesproducebetaradiationastheyrapidlydecay,withtheexceptionof
Cs-135,whichhasahalf-lifeof3×10 6 yr,whichmakesitausefulresearchtool.Cs-137, withahalf-lifeof33years,producesbothbetaandgammaradiation.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=18
s2,p6,d10
5-O=8
s2,p6
6-P=1
s1
Properties Like the other alkali metals, cesium is a soft-solid silvery metal, but much softer than the
others. It is the least electronegative and most reactive of the Earth metals. Cesium has an oxidation state of +1, and because its atoms are larger than Li, Na, and K atoms, it readily gives up its single outer valence electron. The single electron in the P shell is weakly attached to its nucleus and thus available to combine with many other elements. It is much too reactive to be found in its metallic state on Earth.
Cs has a melting point of 29°C, which is lower than the body temperature of humans (37°C), and thus a chunk of cesium will melt in a person’s hand with disastrous results. Since it reacts with moisture on skin as well as with the air to release hydrogen, it will burn vigor- ously through the palm of one’s hand.
Cesium’s boiling point is 669.3°C and its density is 1.837 g/cm 3 . Mercury is the only metal with a lower melting point than cesium. It is extremely dangerous when exposed to air, water, and organic compounds or to sulfur, phosphorus, and any other electronegative elements. It must be stored in a glass container containing an inert atmosphere or in kerosene.
Cesium reacts with water in ways similar to potassium and rubidium metals. In addition to hydrogen, it forms what is known as superoxides, which are identified with the general formula CsO 2. When these superoxides react with carbon dioxide, they release oxygen gas, which makes this reaction useful for self-contained breathing devices used by firemen and
Guide to the Elements | 61 Characteristics
Cesium is located between rubidium and francium in group 1 of the periodic table. It is the heaviest of the stable alkali metals and has the lowest melting point. It is also the most reactive of the alkali metals.
Cesium will decompose water, producing hydrogen, which will burn as it is liberated from
H 2 O. Cesium is extremely dangerous to handle and will burn spontaneously or explode when exposed to air, water, and many organic compounds.
AbundanceandSource The stable form of Cs-133 is the 48th most abundant element on Earth, but because it is
so reactive, it is always in compound form. The Earth’s crust contains only about 7 ppm of Cs-133. Like the other alkali metals, it is found in mixtures of complex minerals. Its main
source is the mineral pollucite (CsAlSi 2 O 6 ). 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 main process to produce cesium is to finely grind its ores and then heat the mix to about 600°C along with liquid sodium, which produces an alloy of Na, Cs, and Ru, which are separated by fractional distillation. Cesium can also be produced by the thermochemical reduction of a mixture of cesium chloride (CsCl) and calcium (Ca).
History In 1860 Gustav Kirchhoff and Robert Bunsen discovered cesium while experimenting
with their spectroscope. While analyzing mineral water, they identified diffraction lines for sodium, lithium, calcium, and strontium. As they removed these elements from their samples, they noticed two bright blue lines indicating the discovery of a new element, and they named cesium for the blue color of the lines seen through the prism in their spectroscopic analysis of the heated sample. Kirchhoff and Bunsen were the first to publish the technology of spec- troscopy, in a book titled Chemical Analysis through Observation of the Spectrum. Their system became a valuable asset in the identification of many other elements.
CommonUses Because of some of its longer-lived isotopes, cesium has become valuable for its ability to
produce a steady stream of beta particles (β) as electrons. Light is strong enough to “knock off” electrons from cesium, which makes this phenom- enon useful as a coating for photoelectric cells and electric eye devices. Cesium iodide (CsI) is used in scintillation counters (Geiger counters) to measure levels of external radiation. It is also useful as a “getter” to remove air molecules remaining in vacuum tubes.
In 1960 the International Committee of Weights and Measures selected radioactive cesium- 137 (with a half-life of about 33 years) as the standard for measuring time. They equated the second with the radiation emitted by a Cs-137 atom that is excited by a small energy source. Thus, the second is now defined as 9,192,631,770 vibrations of the radiation emitted by an atom of Cs-137. There are about 200 atomic clocks around the world that collaborate their