The atomic mass of an atom do not contain electrons because Electrons do not exist in all atoms.
Electrons are so small that it does not affect the mass of an atom.
Electrons are do not contain mass.
Electrons are not as important as protons and neutrons.

Electrons are so small that it does not affect the mass of atom .

Explanation:

Electrons are much smaller in mass than protons, weighing only 9.11 × 10^-28 grams, or about 1/1800 of an atomic mass unit. Therefore, they do not contribute much to an element's overall atomic mass.

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