The '''mass number''' ('''''A'''''), also called ''atomic mass number'' or ''[[nucleon]] number'', is the number of [[proton]]s and [[neutron]]s in an [[atomic nucleus]]. The mass number is unique for each [[isotope]] of an element and is written either after the element name or as a superscript to the left of an element's symbol. For example, carbon-12 (<sup>12</sup>C) has 6 protons and 6 neutrons. The full isotope symbol would also have the [[atomic number]] ('''''Z''''') as a subscript to the left of the element symbol directly below the mass number: <math>{}_{6}^{12}\mathrm{C}</math>. Note that this is redundant, as there is a one-to-one mapping between atomic number and element symbol, so it is rarely used, except when we want to clarify the number of protons in a nucleus, e.g. in atomic reactions.
The difference between the mass number and the atomic number (<math>A-Z</math>) gives the number of neutrons in a given nucleus.
