The mass number of an atom is determined by

The mass number (A) of an atom is defined as the total number of protons and neutrons present in its nucleus. Protons and neutrons, collectively known as nucleons, account for nearly all of an atom's mass because electrons have a negligible mass in comparison [1]. While the atomic number (Z) represents only the number of protons, the mass number is calculated by adding the number of neutrons to the number of protons. This relationship is expressed by the formula: Mass Number = Number of Protons + Number of Neutrons. Although neutral atoms contain an equal number of protons and electrons, the electron count is not used to determine the mass number [1]. Isotopes of an element are characterized by having the same number of protons but different numbers of neutrons, resulting in different mass numbers [1].

Sources

1. [1] https://www.khanacademy.org/science/biology/history-of-life-on-earth/radiometric-dating/a/atomic-number-atomic-mass-and-isotopes-article