Question

1. what is the mass number of an atom?

Explanation:

The mass number of an atom is defined as the sum of the number of protons and neutrons in the nucleus of that atom. Protons and neutrons are the sub - atomic particles that contribute significantly to the mass of an atom (electrons have a negligible mass compared to protons and neutrons).

Answer:

The mass number of an atom is the sum of the number of protons and neutrons in its nucleus.