What is the mass number of an atom equal to?

The mass number of an atom is defined as the total count of protons and neutrons present in the nucleus. This is significant because both protons and neutrons contribute to the overall mass of the atom, while electrons, which are much lighter, do not significantly affect the mass number.

In the context of atomic structure, protons determine the element and its position on the periodic table, while neutrons influence the stability of the nucleus. Atoms of the same element can have different numbers of neutrons, leading to different isotopes, but the mass number remains the sum of protons and neutrons. This concept is fundamental in understanding isotope composition, nuclear reactions, and atomic mass calculations, making it essential knowledge in chemistry.