The mass number of an atom is the total count of protons and neutrons in its nucleus. Protons are positively charged particles, while neutrons have no charge. The mass number helps identify different isotopes of an element, which have the same number of protons but different numbers of neutrons. For example, the element carbon has an atomic number of 6, meaning it has 6 protons. If a carbon atom has 6 neutrons, its mass number is 12. However, if it has 7 neutrons, its mass number becomes 13, indicating a different isotope of carbon.