Question

1. what is the mass number equal to?

number of protons plus number of neutrons
number of electrons plus number of neutrons
number of electrons plus number of protons

Explanation:

In atomic structure, the mass number of an atom is defined as the sum of the number of protons and neutrons in its nucleus. Electrons have negligible mass compared to protons and neutrons and are not included in the calculation of the mass number.

Answer:

Number of protons plus number of neutrons