Mass number

Nuclear physics
Nucleus Nucleons p n Nuclear matter Nuclear force Nuclear structure Nuclear reaction
Models of the nucleus Liquid drop Nuclear shell model Interacting boson model Ab initio
Nuclides' classification Isotopes – equal Z Isobars – equal A Isotones – equal N Isodiaphers – equal N − Z Isomers – equal all the above Mirror nuclei – Z ↔ N Stable Magic Even/odd Halo Borromean
Nuclear stability Binding energy p–n ratio Drip line Island of stability Valley of stability Stable nuclide
Radioactive decay Alpha α Beta β 2β 0v β+ K/L capture Isomeric Gamma γ Internal conversion Spontaneous fission Cluster decay Neutron emission Proton emission Decay energy Decay chain Decay product Radiogenic nuclide
Nuclear fission Spontaneous Products pair breaking Photofission
Capturing processes electron 2× neutron s r proton p rp
High-energy processes Spallation by cosmic ray Photodisintegration
Nucleosynthesis and nuclear astrophysics Nuclear fusion Processes: Stellar Big Bang Supernova Nuclides: Primordial Cosmogenic Artificial
High-energy nuclear physics Quark–gluon plasma RHIC LHC
Scientists Alvarez Becquerel Bethe A. Bohr N. Bohr Chadwick Cockcroft Ir. Curie Fr. Curie Pi. Curie Skłodowska-Curie Davisson Fermi Hahn Jensen Lawrence Mayer Meitner Oliphant Oppenheimer Proca Purcell Rabi Rutherford Soddy Strassmann Świątecki Szilárd Teller Thomson Walton Wigner
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The mass number (symbol A, from the German word: Atomgewicht, "atomic weight"),^[1] also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is approximately equal to the atomic (also known as isotopic) mass of the atom expressed in atomic mass units. Since protons and neutrons are both baryons, the mass number A is identical with the baryon number B of the nucleus (and also of the whole atom or ion). The mass number is different for each isotope of a given chemical element, and the difference between the mass number and the atomic number Z gives the number of neutrons (N) in the nucleus: N = A − Z.^[2]

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 carbon is carbon-12, or ¹²
C
, which 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: ¹²
₆C
.^[3]