Living Reference Work Entry

Encyclopedia of Geochemistry

Part of the series Encyclopedia of Earth Sciences Series pp 1-5

Date: Latest Version


  • Gray E. BeboutAffiliated withDepartment of Earth and Environmental Sciences, Lehigh University Email author 

Element Data

Atomic Symbol: Cs

Atomic Number: 55

Atomic Weight: 132.90545 u

Isotopes and Abundances:133Cs, ~100 %

1 Atm Melting Point: 28.5 °C

1 Atm Boiling Point: 670.8 °C

Common Valences: 1+

Ionic Radii: 12-fold: 188 pm

Pauling Electronegativity: 0.79

First Ionization Energy: 375.4 kJ/mol

Chondritic (CI) Abundance: 0.188 ppm

Silicate Earth Abundance: 7.7 ppb

Crustal Abundance: 2 ppm

Seawater Abundance: 2 nmol/kg

Core Abundance: ~0


Caesium, a reactive silvery-gold metal belonging to the group IA on the periodic table, is one of the alkali elements and considered as a “lithophile” element as defined by Goldschmidt. Caesium, atomic number 55, has a standard atomic weight of 132.90545 and an electronic configuration of [Xe]6 s1. The lone 6 s electron is relatively easily removed; this first ionization potential of Cs is 375.4 kJ/mol. Caesium is highly ionic in character and the ionic radius of Cs1+ is 188 pm in 12-fold coordination. Caesium has a 1 atm melting point of 28.5 °C ...

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