Cesium
Symbol: Cs
Atomic Number: 55
Atomic Mass: 132.90546 amu (atomic mass units)
Melting Point: 28.5 C (301.65 K, 83.3 F)
Boiling Point: 678.4 C (951.55005 K, 1253.12 F)
Number of Protons: 55
Number of Electrons: 55
Number of Neutrons: 78
Classification: Alkali Metal
Crystal Structure: Cubic
Density @ 293 K: 1.873 grams per cubic centimeter
Color: silver
The element cesium was first recognized in the spectrographic analysis of Durkheim mineral water. Robert Wilhelm Bunsen and Gustav Robert Kirchhoff, two German chemists, noticed two blue lines in this spectrograph when they were examining a water sample in 1860. The name cesium comes from the Latin “caesius” which means sky blue and relates to this spectrum.
The spelling cesium is the American version of the elements’ name. The International Union for Pure and Applied Chemistry (IUPAC) approved spelling, caesium, reflects the elements roots.
Cesium has the second lowest melting point of all metals; only mercury has a lower melting point. It is also the most alkaline and electropositive of all the known elements. The pure metal is silvery-white in color, soft and ductile. Cesium reacts extremely violently with water it will react with ice down to temperatures of minus 116 C. The compound cesium hydroxide is the strongest base known. It will attack glass so cesium hydroxide should not be stored in glass bottles.
It has only one naturally occurring stable isotope that is cesium-133. It has more unstable isotopes than any other element with mass numbers ranging from 112 to 151.
Cesium is most commonly extracted from the mineral ore pollucite. Others ores such as lepidolite also contain cesium. The extraction of cesium is complicated as most cesium ores also contain rubidium which is chemically very similar to cesium. The crushed ore must be heated with sodium to a temperature of 650 C (923.15 K, 1202 F) to first form an alloy. The metals within the alloy are then separated by fractional distillation. The largest deposit of cesium containing ore known is at Bernic Lake, Manitoba, Canada. This deposit contains an estimated 300000 tons of pollucite with up to 20% cesium.
Because of the reactive nature of the element pure cesium is rarely supplied. Instead it is shipped as cesium azide. Pure cesium can be extracted from the cesium azide by heating the compound. Cesium has a number of uses.
* Cesium will react with oxygen and so is often used as a getter to remove traces of gas from vacuum tubes.
* It is used in atomic clocks which have an accuracy of 5 seconds in 300 years.
* It acts as a catalyst in the hydrogenation of some organic compounds.
* Cesium is used in photoelectric cells.
* As it has a high mass and it is easily ionized cesium can be used to power ion engines in spacecraft. One kilogram of cesium in an ion propulsion engine can power a craft in space 140 times further than burning an equivalent amount of solid or liquid fuel.
