Term: ionization energy, \(E_{\text{i}}\)
https://doi.org/10.1351/goldbook.I03199

Definition:
Minimum energy required to eject an electron out of a neutral atom or molecule in its ground state. The adiabatic ionization energy refers to the formation of the molecular ion in its ground vibrational state and the vertical ionization energy applies to the transition to the molecular ion without change in geometry. This quantity was formerly called ionization potential. The second ionization energy of an atom is the energy required to eject the second electron from the atom (energy for the process I03199.png).

Related Terms:
1) ground state (http://doi.org/10.1351/goldbook.G02704).
2) adiabatic (http://doi.org/10.1351/goldbook.A00141).
3) ionization (http://doi.org/10.1351/goldbook.I03183).
4) vertical ionization (http://doi.org/10.1351/goldbook.V06610).
5) ionization potential (http://doi.org/10.1351/goldbook.I03208).

Image: Not defined (https://goldbook.iupac.org/img/inline/I03199.png) 

Source: Green Book, 2nd ed., p. 20 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)

Citation: 'ionization energy, \(E_{\text{i}}\)' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.I03199

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