Why can the mass of an electron can be ignored in a nuclear reaction involving protons and neutrons?

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enotechris | College Teacher | (Level 2) Senior Educator

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Mass of proton: 1.6726 x 10^-27 kg
Mass of neutron: 1.6749 x 10^-27 kg
Mass of electron: 9.11 x 10^-31 kg

Protons and neutrons are very similar in mass; however,
as you can see from the table of masses, an electron is much
less massive, about 1830 times less than the two bigger
subatomic particles.

When a transformation between neutrons and protons in the
nucleus of an atom occurs, a neutron can decay into a proton
and and electron:
n -> p + e-

or a proton can transform into an neutron and emit a positron:
p -> n + e+

In both of these equations, the electron (or positron) emitted
has lots of kinetic energy, which is more important to account
for in balancing the energies and masses on either side of the
equations. See link for interesting graphics:

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