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Caesium is a a metallic chemical element (symbol Cs) with an atomic number of 55.


Name, Symbol, Number caesium, Cs, 55
Chemical series alkali metals
Group, Period, Block 1, 6, s
Appearance silvery gold
Standard atomic weight 132.9054519(2) g·mol-1
Electrons per shell 2, 8, 18, 18, 8, 1
Physical properties
Phase solid
Density (near r.t.) 1.93 g·cm-1
Liquid density at m.p. 1.843 g·cm-1
Melting point 301.59K
(28.44°C, 83.19°F)
Boiling point 944K
(671°C, 1240°F)
Critical point 1938 K, 9.4 MPa
Heat of fusion 2.09 kJ·mol-1
Heat of vaporization 63.9 kJ·mol-1
Heat capacity (25°C) 32.210 J·mol-1·K-1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 418 469 534 623 750 940
Atomic properties
Crystal structure body centered cubic
Oxidation states 1
(strongly basic oxide)
Electronegativity 0.79 (Pauling scale)
Ionization energies 1st: 375.7 kJ/mol
2nd: 2234.3 kJ/mol
3rd: 3400 kJ/mol
Atomic radius 260 pm
Atomic radius (calc.) 298 pm
Covalent radius 225 pm
Magnetic ordering no data
Electrical resistivity (20°C) 205 nΩ·m
Thermal conductivity (300K) 35.9 W·m-1·K-1
Thermal expansion (25°C) 97 µm·m-1·K-1
Young's modulus 1.7 GPa
Bulk modulus 1.6 GPa
Mohs hardness 0.2
Brinell hardness 0.14 MPa
CAS registry number 7440-46-2


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