Mass number


The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. The mass number is different for each different isotope of a chemical element. The mass number is written either after the element name or as a superscript to the left of an element’s symbol.

Related formulas


AMass number (protons and neutrons) (dimensionless)
ZAtomic number (protons) (dimensionless)
NNumber of neutrons (dimensionless)