Atomic mass number

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.
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The number of protons and neutrons in an atomic nucleus. It is usually denoted by the symbol, A. This integer number must not be confused with atomic mass, which is the actual mass expressed in atomic mass units . See also atomic number .