The mass nummer (A), an aa cried atomic mass nummer or nucleon nummer, is the tot nummer o protons an neutrons (thegither kent as nucleons) in an atomic nucleus. Acause protons an neutrons baith are baryons, the mass nummer A is identical wi the baryon nummer B as o the nucleus as o the whole atom or ion. The mass nummer is different for each different isotope o a chemical element. This is nae the same as the atomic nummer (Z) which denotes the nummer o protons in a nucleus, an sicweys uniquely identifies an element. Hence, the difference atween the mass nummer an the atomic nummer gies the nummer o neutrons (N) in a gien nucleus: N=A−Z.[1]

The mass nummer is written aither efter the element name or as a superscript tae the left o an element's seembol. For ensaumple, the maist common isotope o carbon is carbon-12, or 12
, which haes 6 protons an 6 neutrons. The full isotope seembol wad an aa hae the atomic nummer (Z) as a subscript tae the left o the element seembol directly ablo the mass nummer: 12
.[2] This is technically redundant, as each element is defined bi its atomic nummer, sae it is eften omittit.


  1. "How many protons, electrons and neutrons are in an atom of krypton, carbon, oxygen, neon,platnum, gold, etc...?". Thomas Jefferson National Accelerator Facility. Retrieved 2008-08-27.
  2. "Elemental Notation and Isotopes". Science Help Online. Retrieved 2008-08-27.