Caesium: properties of free atoms

Caesium atoms have 55 electrons and the shell structure is

The ground state electron configuration of ground state gaseous neutral caesium is  [Xe].6s1 and the term symbol is  2S1/2.

Atomic spectrum


A representation of the atomic spectrum of caesium.

Electronic configuration of neutral atomic caesium

Electronic configuration of caesium

The electronic configuration of caesium.

Kossel shell structure of caesium

The Kossel shell structure of caesium.

Ionisation Energies and electron affinity

The electron affinity of caesium is 45.5 kJ mol‑1. The ionisation energies of caesium are given below.

Ionisation energies of caesium
Ionisation energy number Enthalpy / kJ mol‑1

Effective Nuclear Charges

The following are "Clementi-Raimondi" effective nuclear charges, Zeff. Follow the hyperlinks for more details and for graphs in various formats.

Effective nuclear charges for caesium
2s40.51 2p50.82  
3s36.38 3p36.58 3d40.98  
4s27.04 4p25.86 4d22.84 4f(no data)
5s15.44 5p13.65 5d(no data)  
6s6.36 6p(no data)  


These effective nuclear charges, Zeff, are adapted from the following references:

  1. E. Clementi and D.L.Raimondi, J. Chem. Phys. 1963, 38, 2686.
  2. E. Clementi, D.L.Raimondi, and W.P. Reinhardt, J. Chem. Phys. 1967, 47, 1300.

Electron binding energies

Electron binding energies for caesium. All values of electron binding energies are given in eV. The binding energies are quoted relative to the vacuum level for rare gases and H2, N2, O2, F2, and Cl2 molecules; relative to the Fermi level for metals; and relative to the top of the valence band for semiconductors.
Label Orbital eV [literature reference]
K 1s35985 [1]
L I2s5714 [1]
L II2p1/25359 [1]
L III2p3/25012 [1]
M I3s1211 [2, values derived from reference 1]
M II3p1/21071 [2]
M III3p3/21003 [2]
M IV3d3/2740.5 [2]
M V3d5/2726.6 [2]
N I4s232.3 [2]
N II4p1/2172.4 [2]
N III4p3/2161.3 [2]
N IV4d3/279.8 [2]
N V4d5/277.5 [2]
N VI4f5/2 -
N VII4f7/2 -
O I5s22.7
O II5p1/214.2 [2]
O III5p3/212.1 [2]


I am grateful to Gwyn Williams (Jefferson Laboratory, Virginia, USA) who provided the electron binding energy data. The data are adapted from references 1-3. They are tabulated elsewhere on the WWW (reference 4) and in paper form (reference 5).


  1. J. A. Bearden and A. F. Burr, "Reevaluation of X-Ray Atomic Energy Levels," Rev. Mod. Phys., 1967, 39, 125.
  2. M. Cardona and L. Ley, Eds., Photoemission in Solids I: General Principles (Springer-Verlag, Berlin) with additional corrections, 1978.
  3. Gwyn Williams WWW table of values
  4. D.R. Lide, (Ed.) in Chemical Rubber Company handbook of chemistry and physics, CRC Press, Boca Raton, Florida, USA, 81st edition, 2000.
  5. J. C. Fuggle and N. Mårtensson, "Core-Level Binding Energies in Metals," J. Electron Spectrosc. Relat. Phenom., 1980, 21, 275.

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