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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 protons and neutrons (also defined as a less commonly known term,nucleons) 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: ${}_{6}^{12}$. 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.