mass number The number of protons and neutrons in one atom of a particular isotope of an element. Mass number is also known as the nucleon number since protons and neutrons are collectively known as nucleons. Although all atoms of a given element have the same atomic number, their mass number can differ depending on the number of neutrons they contain. Related entries atomic mass atomic mass unit Related categories • ATOMIC AND NUCLEAR PHYSICS PHYSICAL CHEMISTRY Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) |