The mass number of an atom is the total count of protons and neutrons in its nucleus. It is represented by the symbol \(A\). The mass number provides an indication of the mass of the atom, as protons and neutrons contribute significantly to the atomic mass, while electrons have negligible mass in comparison. For example, if an atom has 6 protons and 6 neutrons, its mass number would be \(6 + 6 = 12\).
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The mass number is the total number of nucleons, namely protons plus neutrons, in an atomic nucleus:It is an exact positive integer with no units, rather than a measured mass. Carbon-12 has because its atomic nucleus contains six protons and six neutrons. No improvement in a mass measurement can turn that count into or .[ref]
The left superscript in denotes mass number , not an atomic mass of exactly . For neon-22, the displayed reference atomic mass is about . Even a weighted mean of mass numbers, such as for the illustrative neon population below, is a population average, not the mass number of an individual atomic nucleus.[ref]