Ionization Potential Or Ionization Energy

The amount of energy required to remove the most loosely bound electron of the outermost shell (i.e. the outermost electron) from one mole of an isolated gaseous atom of an element in its ground state to produce a cation is known as ionization energy of that element. Ionization energy is generally expressed in electron volts, so it is also known as ionization potential. Energy required for the removal of first, second and the third electron from the gaseous atom is called first, second and third ionization energy respectively.

Chemistry CBSE Class 12 Notes

  • 2p and 3p electrons respectively. More energy will be required to separate the electrons from 2s and 3s orbitals.
  • Shielding or screening effect: The shielding or screening effect increases if the number of electrons in the inner shells between the nucleus and the outermost electrons increases. This results in decreases of force of attraction between the nucleus and the outermost electron and lesser energy is required to separate the electron. Thus the value of I.P. decreases.
    Ionization potential ∝ Chemistry CBSE Class 12 Notes
  • Stability of half-filled and fully-filled orbitals: According to Hund’s rule the stability of half filled and completely filled degenerate orbitals is comparatively high. So comparatively more energy is required to separate the electron from the atoms having half filled and full filled electronic configuration.

For example:

  • Removal of electron is comparatively difficult from the half filled configuration of N (Z=7, Is22s22px12py1pz1).
  • The ionization potential of inert gases is very high due to stable ns2np6 electronic configurations.


  • For normal elements: On moving from left to right in a period, value of ionization potential of elements increases because effective nuclear charge also increases.


Chemistry CBSE Class 12 Notes
Chemistry CBSE Class 12 Notes

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