The mass number is called a mass number because it represents the total number of protons and neutrons in the nucleus of an atom, which collectively contribute to the atom’s mass. Protons have a positive charge and are found in the nucleus, while neutrons are neutral particles that also reside in the nucleus. Electrons, which have a negligible mass compared to protons and neutrons, orbit around the nucleus and do not contribute to the mass number.
Since the mass of an atom is primarily determined by its nucleus, the mass number provides a convenient way to quantify the mass of different isotopes of an element. For example, an atom of carbon with six protons and six neutrons has a mass number of 12. This designation helps in distinguishing between isotopes, which are atoms of the same element with different mass numbers due to varying numbers of neutrons. Understanding the mass number is fundamental in studying atomic structure, nuclear reactions, and various applications in chemistry and physics.