mass number (A)

The mass number (A) is the number of protons and neutrons in one atom of a particular isotope of an element. The mass number is written either after the element name or as a superscript to the left of an element's symbol. For example, the most common isotope of oxygen is oxygen-16, or 16O, which has 8 protons and 8 neutrons. 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.