The total number of protons and neutrons in a nucleus.
- The sum of the protons and neutrons is called the mass number.
- Various numbers are used to define nuclei: the atomic number, Z, is the number of protons, while the mass number is the sum of the atomic number and the number of neutrons, N.
- The mass number gives you the total number of protons and neutrons in the nucleus of that element.
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Syllabification: mass num·ber
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