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mass number
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average binding energy per nucleon as a function of the mass number A scientific graph plots average nuclear binding energy per nucleon (in MeV) against atomic mass number, showing a sharp rise at low mass numbers, peaking near iron (Fe), then gradually declining for heavier elements. Key isotopes are labeled along the curve.
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mass number

physics
Also known as: atomic mass number, nucleon number
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mass number, in nuclear physics, the sum of the numbers of protons and neutrons present in the nucleus of an atom. The mass number is commonly cited in distinguishing among the isotopes of an element, all of which have the same atomic number (number of protons) and are represented by the same literal symbol; for example, the two best known isotopes of uranium (those with mass numbers 235 and 238) are designated uranium-235 (symbolized 235U) and uranium-238 (238U).