Which of the following best describes the mass number?

The mass number is defined as the total number of protons and neutrons present in the nucleus of an atom. This is because the mass of an atom is primarily concentrated in its nucleus, where protons and neutrons are located, while electrons contribute negligibly to the overall mass.

Protons carry a positive charge and are fundamental in defining the identity of an element, while neutrons also reside in the nucleus and contribute to the mass of the atom but do not affect the charge. Thus, when we refer to the mass number, we are considering both the protons and neutrons because they account for nearly all of the atom's mass.

This understanding is essential in fields like chemistry and nuclear physics, where mass numbers are used to distinguish between different isotopes of an element, which have the same number of protons but different numbers of neutrons.