Caesium - Cs General Information Discovery Caesium was discovered by R. Bunsen and G.R. Kirchoff in 1860 in Heidelberg, Germany. Appearance Caesium is silvery-white, soft and ductile. It is liquid at room temperature. Source Caesium is found in the minerals pollucite and lepidolite. Pollucite is found in great quantities at Bernic Lake, Manitoba, Canada and the USA, and from this source the element can be prepared. However, most commercial production is as a by-product of lithium production. Uses Caesium is little used. It has a great affinity for oxygen and so is used in electron tubes, and it is also used in photoelectric cells and as a catalyst. A more interesting application is the use in atomic clocks which are accurate to 5 seconds in 300 years. Biological Role Caesium has no known biological role. It is non-toxic. General Information Caesium reacts rapidly with oxygen and explosively with water. It also reacts with ice at temperatures above 116K. The metal is characterised by a spectrum containing two bright lines in the blue along with several others in the red, yellow and green. Caesium hydroxide is the strongest base known, and can attack glass.

	Physical Information
	Atomic Number		55
	Relative Atomic Mass (12C=12.000)		132.91
	Melting Point/K		301.6
	Boiling Point/K		951.6
	Density/kg m-3		1873 (293K)
	Ground State Electron Configuration		[Xe]6s1
	Electron Affinity(M-M-)/kJ mol-1		45.5

	Key Isotopes
	nuclide	133Cs	134Cs	135Cs	137Cs
	atomic mass	132.9
	natural abundance	100%	0%	0%	0%
	half-life	stable	2.05 yrs	3x106 yrs	30.23 yrs

Other Information
Enthalpy of Fusion/kJ mol-1		2.09
Enthalpy of Vaporisation/kJ mol-1		66.5
Oxidation States
Cs-I, CsI
Covalent Bonds /kJ mol-1
not applicable

	Ionisation Energies/kJ mol-1
	M - M+	375.7
	M+ - M2+	2420
	M2+ - M3+	3400
	M3+ - M4+	4400
	M4+ - M5+	6000
	M5+ - M6+	7100
	M6+ - M7+	8300
	M7+ - M8+	11300
	M8+ - M9+	12700
	M9+ - M10+	23700