[Latest] How old is Caesium now?

Living Person

Caesium
is

As of April 18, 2026

166 years old
approximately!

* Calculated from Wikipedia data.
Accuracy is not guaranteed.

Caesium, ₅₅Cs

Caesium
Pronunciation	/ˈsiːziəm/ ​(SEE-zee-əm)
Alternative name	cesium (US)
Appearance	pale gold
Standard atomic weight Ar°(Cs)
	132.90545196±0.00000006 132.91±0.01 (abridged)
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: hydrogen and alkali metals
Period	period 6
Block	s-block
Electron configuration	[Xe] 6s
Electrons per shell	2, 8, 18, 18, 8, 1
Physical properties
Phase at STP	solid
Melting point	301.7 K ​(28.5 °C, ​83.3 °F)
Boiling point	944 K ​(671 °C, ​1240 °F)
Density (at 20° C)	1.886 g/cm
when liquid (at m.p.)	1.843 g/cm
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)
Specific heat capacity	242.344 J/(kg·K)
Vapour 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	common: +1 −1
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	primordial
Crystal structure	​body-centred cubic (bcc) (cI2)
Lattice constant	a = 616.2 pm (at 20 °C)
Thermal expansion	92.6×10/K (at 20 °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 'bluish grey', for its spectral colours
Discovery	Gustav Kirchhoff and Robert Bunsen (1860)
First isolation	Carl Setterberg (1882)
Isotopes of caesium v e
Main isotopes Decay Isotope abun­dance half-life (t1/2) mode pro­duct Cs synth 9.69 d ε Xe Cs 100% stable Cs synth 2.0650 y β Ba ε Xe Cs trace 1.33×10 y β Ba Cs synth 30.04 y β Ba
Category: Caesium view talk edit | references

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