What does the mass number represent?

The mass number of an atom represents the total number of protons and neutrons in its nucleus. Protons and neutrons are collectively known as nucleons, and their combined count gives a measure of the atom's mass.

Protons contribute positively to the mass number and are located in the nucleus, while neutrons are neutral particles that also reside in the nucleus and further add to the mass of the atom. The number of protons determines the atomic number and thus the identity of the element, while neutrons can vary in number, resulting in different isotopes of that element. Therefore, the mass number provides a direct indication of the sum of these two fundamental particles, making it a crucial concept in understanding atomic structure and stability.

In contrast, options that refer only to the number of protons, neutrons, or electrons provide incomplete information about what the mass number encompasses.