Mass Number Definition and Examples

close up of chlorine on periodic table

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Mass number is an integer (whole number) equal to the sum of the number of protons and neutrons of an atomic nucleus. In other words, it is the sum of the number of nucleons in an atom. Mass number is often denoted using a capital letter A.

Contrast this with the atomic number, which is simply the number of protons.

Electrons are excluded from the mass number because their mass is so much smaller than that of protons and neutrons that they don't really affect the value.


3717Cl has a mass number of 37. Its nucleus contains 17 protons and 20 neutrons.

The mass number of carbon-13 is 13. When a number is given following an element name, this is its isotope, which basically states the mass number. To find the number of neutrons in an atom of the isotope, simply subtract the number of protons (atomic number). So, carbon-13 has 7 neutrons, because carbon has atomic number 6.

Mass Defect

Mass number only gives an estimate of isotope mass in atomic mass units (amu).The isotopic mass of carbon-12 is correct because the atomic mass unit is defined as 1/12 of the mass of this isotope. For other isotopes, mass is within about 0.1 amu of the mass number. The reason there is a difference is because of mass defect, which occurs because neutrons are slightly heavier than protons and because the nuclear binding energy is not constant between nuclei.

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Helmenstine, Anne Marie, Ph.D. "Mass Number Definition and Examples." ThoughtCo, Aug. 27, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 27). Mass Number Definition and Examples. Retrieved from Helmenstine, Anne Marie, Ph.D. "Mass Number Definition and Examples." ThoughtCo. (accessed June 5, 2023).