Molecular Ion: Examples, Formation, Mass, Properties


“An ion formed by removing (positive ion), or adding (negative ion), one or more electrons
from a molecule M to form M• or M•• ”


There are some examples of molecular ions

  • Acetylide
  • C2−2
  • Ethoxide or ethanolate
  • C2H5O−
  • Acetate or ethanoate
  • CH3COO− or C2H3O−2
  • Benzoate
  • C6H5COO−or C7H5O−2
  • Citrate
  • C6H5O3−7
  • Carbonate
  • CO2−3
  • Oxalate
  • C2O2−4
  • Cyanide
  • CN−


There are six ions in the mass spectrum of saturated bismuth oxide vapour of Bi4O5, Bi4O4, Bi4O2, Bi3O4, Bi2O2, and Bi2O. The first three ions have IE 9.1, 10.0, and 10.3 eV and are treated as fragments; the next to have history.


Michael Faraday (1791-1867) introduced the term ‘ion’ in 1834 for the then-unknown species that goes from one electrode to the other through an aqueous medium. He wasn’t sure of the nature of the actual ions, but when two electrodes were placed in an aqueous solution, a current could travel between the electrodes. He knew there was something in the solution to make that possible.

Later, Svante Arrhenius (1859-1927) confirmed this theory. He was a many-talented Swede who received the 1903 Nobel prize for chemistry and who (among his many achievements) first suggested the “greenhouse effect.” Arrhenius proposed in 1884 that when a compound like a table salt, NaCl (sodium chloride), was dissolved in water, it broke up into electrically charged “ions” Na+ and Cl-. Electric forces made Na+ ions move in one direction, Cl- ions in the opposite one, and that was how the electric current was carried. E 7.5, 7.9, and 7.5 eV and are considered to be molecular.

The Formation of Molecular ions

When the vaporised organic sample passes into the ionisation chamber of a mass spectrometer, it is bombarded by a stream of electrons. These electrons have a high enough energy to knock an electron off an organic molecule to form a positive ion. This type of ion is called the molecular ion.

Mass of molecular ion

The mass of the molecular ion corresponds to the molecule’s nominal or monoisotopic mass.
The mass of the electron added or lost is usually inconsequential. The mass of a molecular ion is the sum of the relative masses of the most abundant naturally occurring isotopes of the various atoms comprising the molecule.


• Molecular ion is also known as Polyatomic ion
• Polyatomic ions are ions that consist of more than one atom.
• The atoms in a polyatomic ion are usually covalently bonded to one another, and therefore stay together as a single, charged unit.

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