Question

question
the mass number of an atom is defined as the ____________
answer
number of neutrons in the nucleus
number of protons plus the number of neutrons in the nucleus
number of protons plus neutrons plus electrons in an atom
total number of all electrons in the electron shells
i dont know yet

Explanation:

The mass number of an atom is the sum of the number of protons and neutrons in the nucleus. Electrons have negligible mass compared to protons and neutrons and are not included in the mass - number calculation.

Answer:

number of protons plus the number of neutrons in the nucleus