Mass Number (A)

The mass number, symbol A, also called atomic mass number (not to be confused with atomic number (Z)) or nucleon number, is the number of nucleons (protons and neutrons) 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.

Carbon-12 (12C) has 6 protons (as do ALL isotopes of carbon) and 6 neutrons, 6+6 = 12.

Lead-207 (207Pb) has 82 protons (as do ALL isotopes of lead) and 125 neutrons, 82+125 = 207.

See Also

Atomic Number (Z).