Nucleons are the protons and neutrons inside an atom’s nucleus. The number of protons identifies the element; the number of neutrons can vary. Add those two counts to get the nucleus’s mass number: A = Z + N.
What counts as a nucleon?
A nucleon is either a proton or a neutron that is part of an atomic nucleus. The U.S. Department of Energy defines it as “a constituent particle of the atomic nucleus, either a neutron or a proton” in its Nuclear Physics Study Guide.
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- Protons carry one positive elementary charge.
- Neutrons have no electric charge.
Protons and neutrons have approximately the same mass. Electrons are not nucleons: they occupy the space around the nucleus and are not included in the nucleus’s nucleon count. The nucleus itself is a very small region at the center of the atom. OpenStax’s overview of nuclear properties explains these basic components and counts.
How do proton, neutron, and mass numbers fit together?
Three symbols keep the counts straight. Z is the number of protons, N is the number of neutrons, and A is the mass number—the total number of nucleons. Therefore, A = Z + N.
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- Z tells you the element. Every atom of a given element has the same proton count.
- N tells you how many neutrons are present. This count can differ among atoms of the same element.
- A counts protons and neutrons together. Electrons are excluded.
For example, carbon-12 has six protons and six neutrons. Its atomic number is Z = 6, its neutron number is N = 6, and its mass number is A = 12.
Reading nuclide notation
A nuclide is a particular kind of nucleus, specified by its proton and neutron counts. In the notation AZX, X is the element symbol, the superscript A is the mass number, and the subscript Z is the atomic number. The neutron count can be found by subtracting: N = A − Z. For carbon-12, 126C, that gives six neutrons.
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What are isotopes?
Isotopes are nuclei of the same element that have the same number of protons but different numbers of neutrons. Since the proton count stays fixed, the element stays the same; changing the neutron count changes the isotope and its mass number.
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| Hydrogen isotope | Protons | Neutrons | Mass number |
|---|---|---|---|
| Hydrogen-1 | 1 | 0 | 1 |
| Deuterium (hydrogen-2) | 1 | 1 | 2 |
| Tritium (hydrogen-3) | 1 | 2 | 3 |
Hydrogen-1 has no neutron, so it is inaccurate to say that every hydrogen nucleus contains one. The International Atomic Energy Agency’s introductory nuclear-physics material also uses the hydrogen isotopes to illustrate how nuclei differ.
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Is mass number the same as measured mass?
No. Mass number is an integer count of nucleons, not the measured mass of an isotope. The measured mass can differ from the sum of the masses of the separate protons and neutrons because energy binds the nucleus together. For calculations involving atomic masses, electron masses may also need to be accounted for; OpenStax’s binding-energy explanation discusses that distinction.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What does nuclear binding energy mean?
Binding energy is the energy required to separate a nucleus completely into its constituent protons and neutrons. A bound nucleus has less mass than those separated constituents; the difference is called the mass defect. The relationship is expressed by Einstein’s equation E = Δmc², where Δm is the mass defect and c is the speed of light.
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Binding energy per nucleon is a useful way to compare nuclei when discussing nuclear stability, but it does not replace identifying the specific isotope. OpenStax’s worked discussion of nuclear structure and stability places the greatest binding energy per nucleon near mass number 56. Its iron-56 example calculates 8.820 MeV per nucleon. These are a broad trend and one worked example, not a claim that every nucleus near that mass has identical stability.
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