An Etymological Dictionary of Astronomy and Astrophysics

The total number of → protons and → neutrons in the → atomic nucleus (symbol A). The mass number is written either after the → chemical 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.

See also: → mass; → number.