Mass Number (m)
From Mass Spec Terms
Orange Book Entry
The sum of the total number of protons and neutrons in an atom, ion or molecule.
Gold Book Entry
Total number of heavy particles (protons and neutrons jointly called nucleons) in the atomic nucleus. Also called nucleon number. Symbol m in mass spectrometry.
Proposed New Entry
Total number of nucleons (protons and neutrons) in an atom or molecule. Should be called nucleon number instead of mass number. Currently the symbol m is widely used in mass spectrometry. To avoid confusion between the nucleon number and mass, the symbol n should be used for the nucleon number.
The nucleon number n should not be used in mass spectra because:
- mass spectrometers measure mass/charge and not the number of nucleons nor the number of nucleons/charge
- the number of nucleons n is a natural number (whole number larger than zero) whereas mass spectrometers measure fractional masses.
