Respuesta :

Answer:

2. For any given isotope, the sum of the numbers of protons and neutrons in the nucleus is called the mass number.

3. in fact, electron mass is so small that it is not counted in an atom's mass

Explanation:

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You can calculate the mass number by finding the sum of the neutrons and protons. An electron is not counted when calculating mass because it is so small it is nearly massless.