Caesium

Caesium or cesium^{[note 1]} is a chemical element wi seembol Cs an atomic nummer 55. It is a saft, sillery-gowd alkali metal wi a meltin pynt o 28 °C (82 °F), which maks it ane o anly five elemental metals that are liquid at or near ruim temperatur.^{[note 2]} Caesium is an alkali metal an haes pheesical an chemical properties seemilar tae those o rubidium an potassium. The metal is extremely reactive an pyrophoric, reactin wi watter even at −116 °C (−177 °F). It is the least electronegative element havin a stable isotope, caesium-133. Caesium is mined maistly frae pollucite, while the radioisotopes, especially caesium-137, a fission product, are extractit frae waste produced bi nuclear reactors.

Caesium, ₅₅Cs

Caesium
Pronunciation	/ˈsiːziəm/ ​(SEE-zee-əm)
Appearance	sillery gowd
Standard atomic weight Ar, std(Cs)	7002132905451960000♠132.90545196(6)[1]
Caesium in the periodic cairt
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 Airn Cobalt Nickel Capper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Siller (element) 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 Gowd Mercur (element) Thallium Leid (element) 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 Ununtrium Flerovium Ununpentium Livermorium Ununseptium Ununoctium Rb ↑ Cs ↓ Fr xenon ← caesium → barium
Atomic nummer (Z)	55
Group	group 1 (alkali metals)
Period	period 6
Block	s-block
Element category	Alkali metal
Electron confeeguration	[Xe] 6s1
Electrons per shell	2, 8, 18, 18, 8, 1
Pheesical properties
Phase at STP	solit
Meltin pynt	301.59 K ​(28.44 °C, ​83.19 °F)
Bylin pynt	944 K ​(671 °C, ​1240 °F)
Density (near r.t.)	1.93 g/cm3
when liquid (at m.p.)	1.843 g/cm3
Creetical pynt	1938 K, 9.4 MPa
Heat o fusion	2.09 kJ/mol
Heat o vapourisation	63.9 kJ/mol
Molar heat capacity	32.210 J/(mol·K)
Vapour pressur 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[2] strangly basic
Electronegativity	Pauling scale: 0.79
Atomic radius	empirical: 265 pm
Covalent radius	244±11 pm
Van der Waals radius	343 pm
Spectral lines o caesium
Ither properties
Naitural occurrence	primordial
Creestal structur	​body-centred cubic (bcc)
Thermal expansion	97 µm/(m·K) (at 25 °C)
Thermal conductivity	35.9 W/(m·K)
Electrical resistivity	205 n Ω·m (at 20 °C)
Magnetic orderin	paramagnetic[3]
Young's modulus	1.7 GPa
Bulk modulus	1.6 GPa
Mohs haurdness	0.2
Brinell haurdness	0.14 MPa
CAS Nummer	7440-46-2
History
Diskivery	Robert Bunsen an Gustav Kirchhoff (1860)
First isolation	Carl Setterberg (1882)
Main isotopes o caesium
Iso­tope Abun­dance Hauf-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 trace 30.17 y[4] β− 137Ba
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Twa German chemists, Robert Bunsen an Gustav Kirchhoff, discovered caesium in 1860 bi the newly developed method o flame spectroscopy. The first smaa-scale applications for caesium wur as a "getter" in vacuum tubes an in photoelectric cells. In 1967, a specific frequency frae the emission spectrum o caesium-133 wis chosen tae be uised in the definition o the seicont bi the Internaitional Seestem o Units. Syne then, caesium haes been widely uised in atomic clocks.

Syne the 1990s, the lairgest application o the element haes been as caesium formate for drilling fluids. It haes a range o applications in the production o electricity, in electronics, an in chemistry. The radioactive isotope caesium-137 haes a hauf-life o aboot 30 year an is uised in medical applications, industrial gauges, an hydrology. Altho the element is anly mildly toxic, it is a hazardous material as a metal an its radioisotopes present a heich heal risk if released intae the environment.