Caesium

Caesium (or cesium) is the chemical element with the atomic number 55 on the periodic table. Its symbol is Cs.

Caesium,  ₅₅Cs

General properties
Pronunciation	/ˈsiːziəm/ ​(SEE-zee-əm)
Alternative name	cesium (US, informal)
Appearance	pale gold
Standard atomic weight (Ar, standard)	132.90545196(6)
Caesium in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Rb ↑ Cs ↓ Fr xenon ← caesium → barium
Atomic number (Z)	55
Group	group 1 (alkali metals)
Period	period 6
Block	s-block
Element category	alkali metal
Electron configuration	[Xe] 6s1
Electrons per shell	2, 8, 18, 18, 8, 1
Physical properties
Phase at STP	Cs: Solid
Melting point	301.7 K ​(28.5 °C, ​83.3 °F)
Boiling point	944 K ​(671 °C, ​1240 °F)
Density (near r.t.)	1.93 g/cm3
when liquid (at m.p.)	1.843 g/cm3
Critical point	1938 K, 9.4 MPa
Heat of fusion	2.09 kJ/mol
Heat of vaporization	63.9 kJ/mol
Molar heat capacity	32.210 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 418 469 534 623 750 940
Atomic properties
Oxidation states	−1, +1 (a strongly basic oxide)
Electronegativity	Pauling scale: 0.79
Ionization energies	1st: 375.7 kJ/mol 2nd: 2234.3 kJ/mol 3rd: 3400 kJ/mol
Atomic radius	empirical: 265 pm
Covalent radius	244±11 pm
Van der Waals radius	343 pm
Spectral lines of caesium
Other properties
Natural occurrence	Cs: Primordial
Crystal structure	​body-centered cubic (bcc)
Thermal expansion	97 µm/(m·K) (at 25 °C)
Thermal conductivity	35.9 W/(m·K)
Electrical resistivity	205 nΩ·m (at 20 °C)
Magnetic ordering	paramagnetic
Young's modulus	1.7 GPa
Bulk modulus	1.6 GPa
Mohs hardness	0.2
Brinell hardness	0.14 MPa
CAS Number	7440-46-2
History
Naming	from Latin caesius, sky blue, for its spectral colours
Discovery	Robert Bunsen and Gustav Kirchhoff (1860)
First isolation	Carl Setterberg (1882)
Main isotopes of caesium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct 133Cs 100% stable 134Cs syn 2.0648 y ε 134Xe β− 134Ba 135Cs trace 2.3×106 y β− 135Ba 137Cs syn 30.17 y β− 137Ba

Caesium is an alkali metal. Its melting point is low (28 °C). It is extremely reactive. Because of its high reactivity, it is a dangerous chemical. It may set itself on fire (ignite) in air. It explodes on contact with water. It reacts more violently than the other alkali metals with water. Because of this, caesium is stored in mineral oil.

Caesium is a rare element. Since there is little caesium on the Earth, it is rather expensive. The human body does not need caesium. In large amounts, its chemical compounds are mildly poisonous because it is close to potassium, which the body does need.

History

Caesium was first described in 1860, by Gustav Robert Kirchhoff and Robert Wilhelm Bunsen. They were testing mineral water, from Bad Dürkheim. After they separated calcium, strontium, magnesium and lithium, they saw two lines in the "blue" range of the spectrum. Because of these lines, they concluded that in addition to the elements already found, there must be another unknown substance in the mineral water. They named this substance caesium, after the color blue.

Isotopes and compounds

Caesium has at least 39 known isotopes ranging in atomic mass from 112 to 151. Only one of these, ¹³³Cs, is stable. Therefore, the naturally-occurring isotope of caesium is ¹³³Cs, which is not radioactive. ¹³³Cs is used in atomic clocks, its vibration frequency used to define the length of the second. Another isotope, ¹³⁷Cs is not made naturally but is made after nuclear fission has been done. It is very radioactive and used as an industrial gamma ray source.

Caesium forms compounds with many other chemical elements. Caesium formate is used in oil drilling because of its high density.

Reactivity

Caesium is extremely reactive in air and water. Caesium rapidly oxidizes in air and can spontaneously combust (randomly catch on fire) at any moment. For this reason, it must be stored in kerosene or a mineral oil, like other group one elements (Lithium, Natrium, Rubidium, and Francium.) In water, Caesium violently reacts to make Caesium Hydroxide (2CsOH). The Caesium sinks for about one second, then explodes. The explosion is over 50 times the size of the element dropped in the water, and the explosion is enough to break a common Pyrex Beaker, Flask, or Test Tube. You can find a video of the reaction here.

Caesium Media

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  High-purity caesium-133 stored in argon.

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  Caesium crystals (golden) compared to rubidium crystals (silvery)

• Addition of a small amount of caesium to cold water is explosive.

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  27 small grey spheres in 3 evenly spaced layers of nine. 8 spheres form a regular cube and 8 of those cubes form a larger cube.

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  Monatomic caesium halide wires grown inside double-wall carbon nanotubes (TEM image).

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  Cs₁₁O₃ cluster

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  Cs-137 decay diagram

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  Pollucite, a caesium mineral

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  Atomic clock ensemble at the U.S. Naval Observatory