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Why is the number of electrons not included in the atomic mass?

Answer 1

Wyatt Adkins

Because the mass contributed by electrons is dwarfed by the nuclear contribution.

Mass of an electron, #9.10xx10^(-31)*kg#.

Mass of a proton, #1.67xx10^(-27)*kg#.

Three orders of magnitude separate the two. To a first approximation, electronic mass can be disregarded.

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Answer 2

Jose Ayala

The number of electrons is not included in the atomic mass because electrons have negligible mass compared to protons and neutrons, which make up the majority of an atom's mass.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.