Question

in an atom, the mass number of an atom is equal to the number of ---
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Answer 1

Explanation:

The mass of any atom is equal to the mass of the protons, neutrons and electrons that make it up, but 1 electron has about 1/2000 of the mass of a proton, so they don't count for much.

Answer 2

Explanation:

The mass number of an atom is equal to the number of protons plus the number of neutrons that it contains.

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