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Atomic mass number

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Mass number
The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It determines the atomic mass of atoms. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion. The mass number is different for each different isotope of a chemical element. This is not the same as the atomic number (Z) which denotes the 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: N=A-Z.

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ASTRONOMY UNBOUND Dictionary

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Atomic mass number
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 .


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