Question

how is mass number calculated?
○ by subtracting the mass of the protons from the mass number
○ by adding the mass of the protons and neutrons
○ by adding the mass of the electrons and protons
○ by subtracting the mass of the neutrons from the mass number

Explanation:

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

Answer:

by adding the mass of the protons and neutrons