What is the mass number of an atom?

The mass number of an atom is defined as the total number of protons and neutrons present in the nucleus. Protons and neutrons are both nucleons, meaning they reside in the atomic nucleus, and their combined count gives us the mass number. This number is crucial as it determines the isotope of an element, which represents the different forms of an element that vary in neutron count while retaining the same number of protons, or atomic number.

In contrast, the total number of electrons in an atom pertains to the atom's charge but does not factor into the mass number directly. The average mass of all isotopes of an element refers to isotopic abundance and is known as the atomic mass, not the mass number. Lastly, the calculation of the number of protons minus the number of neutrons does not provide a relevant measurement for mass number; rather, it would yield the net difference between these two subatomic particles, which is not a standard atomic characteristic. Therefore, the correct definition of mass number is indeed the total number of protons and neutrons in the nucleus.