Navy Academic Proficiency Test (NAPT) Practice Exam

Mass number is the total count of particles in the nucleus—the protons and the neutrons. These nucleons make up almost all of an atom’s mass, while electrons orbit the nucleus and contribute only a negligible amount to mass, so they aren’t included in the mass number. For example, carbon-12 has 6 protons and 6 neutrons, giving a mass number of 12. Isotopes differ in the number of neutrons, which changes the mass number, while the number of protons stays the same. So the sum is protons plus neutrons.