The mass number (A), also called atomic mass number (not to be confused with atomic number ('Z) which denotes the number of protons in a nucleus) 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. For example, carbon-12 (¹²C) 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: . 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, such as in atomic reactions. It has been suggested that List of elements by atomic number be merged into this article or section. ... In physics a nucleon is a collective name for two baryons: the neutron and the proton. ... In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... This article or section does not adequately cite its references or sources. ... The nucleus of an atom is the very small dense region, of positive charge, in its centre consisting of nucleons (protons and neutrons). ... Isotopes are any of the several different forms of an element each having different atomic mass (mass number). ... Carbon 12 is a stable isotope of the element carbon. ... It has been suggested that List of elements by atomic number be merged into this article or section. ...

The difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: N=A−Z.

For example: Carbon-14 is created from Nitrogen-14 with seven protons (p) and seven neutrons via a cosmic ray interaction which transmutes 1 proton into 1 neutron. Thus the atomic number decreases by 1 (Z: 7→6) and the mass number remains the same (A = 14), however the number of neutrons increases by 1 (n: 7→8). Carbon-14 is the radioactive isotope of carbon discovered February 27, 1940, by Martin Kamen and Sam Ruben. ... Nitrogen-14 is a stable, non-radioactive isotope of the nitrogen element. ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ...

Before: Nitrogen-14 (7p, 7n)

After: Carbon-14 (6p, 8n).

This should not be confused with the relative atomic mass which is the average abundance atomic mass number of the differing isotopes found. For instance, there are two isotopes of chlorine: Chlorine-35 and Chlorine-37. In any given sample of chlorine that has not had any mass separation there will be roughly 75% of chlorine atoms which are chlorine-35 and only 25% of chlorine atoms which are Chlorine-37. This gives chlorine a relative atomic mass of 35.5 (actually 35.4527 g/mol). The atomic mass (ma) is the mass of an atom at rest, most often expressed in unified atomic mass units. ... MOL can refer to: Manned Orbiting Laboratory Method of Levels (psychotherapy) MOL, (Hungarian Oil and Gas Public Limited Company [1]) Method of Lines Mac-on-Linux, a software environment for PowerPC Linux which can run Mac OS (or Mac OS X) and its applications This page concerning a three-letter...