As far as I know, the mass number of an atom means the amount of protons and neutrons it has. For example, the mass number of Sodium (Na) is 23 since it has 11 protons and 12 neutrons. Then how electron, proton and neutron have mass numbers? Like somewhere I read that the mass number of electron is 0.00055 amu. What does this mean?

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    $\begingroup$ what is exactly your question ? $\endgroup$
    – Naveen V
    Mar 16, 2023 at 18:05
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    $\begingroup$ mass number is a property of atoms/ions they do not belong to individual substituents like electron , proton or neutron $\endgroup$
    – Naveen V
    Mar 16, 2023 at 18:11
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    $\begingroup$ for your recent edit , mass of electron and mass number of an element are totally different things , mass number is just the amount of nucleons present in the atom/ion it has very little to do with the actual mass of an electron , the actual mass is given in amu (now 'u') measurement and still as the answer defined mass number of electron is $0$ not the actual mass of an electron $\endgroup$
    – Naveen V
    Mar 17, 2023 at 8:48

1 Answer 1


It makes sense to also assign mass numbers to the sub-atomic constituents:

  • A proton has mass number $1$.
  • A neutron has mass number $1$.
  • An electron has mass number $0$.

With this definition you consistently get the mass number of an atom/ion by summing the mass numbers of its constituents. For example: the Na atom and the Na$^+$ ion both have mass number $23$.

Don't confuse these mass numbers from above with the actual masses of the sub-atomic constituents:

  • An electron has mass $0.00055$ u.
  • A free proton has mass $1.0073$ u.
  • A free neutron has mass $1.0087$ u.
  • Protons and neutrons bound inside a nucleus have slightly smaller masses (typically less by $0.008$ u) due to the nuclear binding energy.

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