Title: molecular ion
Long Title: IUPAC Gold Book - molecular ion
DOI: 10.1351/goldbook.M03988
Status: current

Definition
An ion formed by the removal from (positive ions) or addition to (negative ions) a molecule of one or more electrons without fragmentation of the molecular structure. The mass of this ion corresponds to the sum of the masses of the most abundant naturally occurring isotopes of the various atoms that make up the molecule (with a correction for the masses of the electron(s) lost or gained). For example, the mass of the molecular ion of ethyl bromide $\ce{C2H5^{79}Br}$ will be \(2\times 12\) plus \(5\times 1.0078246\) plus 78.91839 minus the mass of the electron (\(m_{\rm{e}}\)). This is equal to \(107.95751\ \rm{u}-m_{\rm{e}}\), \(u\) being the unified atomic mass unit based on the standard that the mass of the isotope $\ce{^{12}C}$ = \(12\ \rm{u}\) exactly.

Related Terms
- fragmentation: https://goldbook.iupac.org//terms/view/F02506
- isotopes: https://goldbook.iupac.org//terms/view/I03331
- unified atomic mass unit: https://goldbook.iupac.org//terms/view/U06554

Sources
- Orange Book, 2nd ed., p. 205 (https://media.iupac.org/publications/analytical_compendium/)
- PAC, 1991, 63, 1541. 'Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)' on page 1549 (https://doi.org/10.1351/pac199163101541)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/M03988/html
- json: https://goldbook.iupac.org/terms/view/M03988/json
- xml: https://goldbook.iupac.org/terms/view/M03988/xml

Citation: Citation: 'molecular ion' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.M03988
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Accessed: 2026-04-20T17:05:06+00:00