The mass number of an atom is the sum of the number of protons and neutrons in its nucleus. It is often denoted by the symbol "A." Protons and neutrons are collectively known as nucleons, and they contribute to the mass of the atom. The mass number represents the total number of nucleons in the atom's nucleus.
For example, if an atom has 6 protons and 6 neutrons in its nucleus, its mass number would be 12 (6 protons + 6 neutrons = 12 nucleons). The mass number is an important characteristic of an atom and is used to identify different isotopes of an element. Isotopes of an element have the same number of protons but different numbers of neutrons, resulting in different mass numbers.