Question

which is equal to the mass number of an atom?
number of protons
protons plus neutrons
number of electrons
protons plus electrons

Explanation:

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

Answer:

protons plus neutrons