Nucleons are the protons and neutrons in an atom’s nucleus. Protons have a positive electric charge, neutrons have no electric charge, and together they account for the nucleus’s mass number. The key relationship is A = Z + N: mass number equals the number of protons plus the number of neutrons.
What does “nucleon” mean?
A nucleon is either of the two kinds of particles that make up an atomic nucleus: a proton or a neutron. 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.
The nucleus is the extremely small region at an atom’s center. Electrons are outside the nucleus, so they are not nucleons and are not counted in the mass number. Protons and neutrons have approximately the same mass, but differ in electric charge: a proton is positive, while a neutron is neutral. OpenStax University Physics describes nuclear composition and notation.
How to count protons and neutrons
Three symbols keep the counts straight:
- Z is the atomic number: the number of protons. It determines the element.
- N is the neutron number: the number of neutrons in the nucleus.
- A is the mass number: the total number of nucleons, so A = Z + N.
A nuclide is a particular kind of nucleus, specified by its proton and neutron counts. Its notation places the mass number A as a superscript and atomic number Z as a subscript next to the element symbol, X. For example, carbon-12 is written with A = 12 and Z = 6; therefore, it has 6 protons and 12 − 6 = 6 neutrons. The mass number is a whole-number count of nucleons, not the isotope’s measured mass. OpenStax’s nuclear-properties chapter explains the notation.
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How isotopes differ
Isotopes are nuclei of the same element with the same number of protons but different numbers of neutrons. Since proton count defines the element, changing the neutron count changes the isotope, not the element.
| Hydrogen isotope | Protons (Z) | Neutrons (N) | Mass number (A) |
|---|---|---|---|
| Hydrogen-1 | 1 | 0 | 1 |
| Deuterium (hydrogen-2) | 1 | 1 | 2 |
| Tritium (hydrogen-3) | 1 | 2 | 3 |
These counts illustrate why it is inaccurate to say that every hydrogen nucleus contains a neutron: ordinary hydrogen-1 has one proton and none. OpenStax also gives hydrogen-3 a 12.32-year half-life and carbon-14 a 5730-year half-life as examples; those figures apply to those isotopes, not to isotopes generally. See OpenStax University Physics and the International Atomic Energy Agency’s Basic Nuclear Physics.
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What holds nucleons together?
Nucleons are bound together in the nucleus. Nuclear 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. Mass defect and binding energy are related by Einstein’s equation, E = Δmc². OpenStax College Physics explains the relationship and the mass accounting used in binding-energy calculations.
Binding energy per nucleon
Dividing a nucleus’s binding energy by its number of nucleons gives binding energy per nucleon, a useful way to compare nuclei when discussing stability. It does not identify an isotope on its own, and similar mass numbers do not mean nuclei have identical stability. OpenStax Chemistry places the broad maximum near mass number 56 and works through iron-56 at 8.820 MeV per nucleon; that is a specific textbook example, not a value for every nucleus near 56. See OpenStax Chemistry.
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Three distinctions to remember
- Nucleon: a proton or neutron in the nucleus; an electron is not a nucleon.
- Element versus isotope: proton count sets the element, while a different neutron count gives a different isotope of that element.
- Mass number versus measured mass: A counts nucleons; it is not the same as an isotope’s measured mass, which reflects the nucleus’s binding.
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