Mass number number of protons in a nucleus, and thus uniquely identifies an element. Hence, the difference between the mass number and the atomic number gives the number of neutrons ("N") in a given nucleus: . 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 , 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:
Standard atomic weight Standard atomic weight The standard atomic weight ("A", a relative atomic mass) is the atomic weight ("A") of a chemical element, as appearing and met in the earthly environment. It reflects the variance of natural isotopes (and so weight differences) of an element. Values are defined by (restricted to) the IUPAC (CIAAW) definition of natural, stable, terrestrial sources. It is the most common and practical atomic weight used, for example to determine molar mass. The specified definition is to use many representative sources (samples) from the Earth, so that the value can widely be used as 'the' atomic weight for