What Is a Nucleon?
Author: Domeny.ForSale editorial team Published
In brief: Nucleon is the collective name for protons and neutrons—the particles that make up atomic nuclei. The number of nucleons in a nucleus is the sum of its protons and neutrons, known as the mass number. Electrons are not nucleons.
Protons and neutrons—two types of nucleon
A proton has a positive electric charge, while a neutron is electrically neutral. Their masses are very similar, although a neutron is slightly heavier than a proton. Both particles are about 1800 times heavier than an electron, the negatively charged particle found outside the nucleus in an atom.
| Particle | Electric charge | Is it a nucleon? |
|---|---|---|
| Proton | Positive | Yes |
| Neutron | Zero | Yes |
| Electron | Negative | No |
The term “nucleon” lets us refer to protons and neutrons collectively when the difference between them is not important, for example when counting the constituents of a nucleus. It does not refer to a third type of particle alongside protons and neutrons.
A proton or neutron remains a nucleon even when it is outside an atomic nucleus. The name identifies the type of particle, not just its location.
How do you count nucleons in a nucleus?
Three symbols are used to describe the composition of a nucleus: Z is the atomic number, or the number of protons; N is the number of neutrons; and A is the mass number, or the total number of nucleons. The relationship between them is simple:
A = Z + N
If you know the mass number and atomic number, you can calculate the number of neutrons using the formula N = A − Z. The number of electrons does not appear in any of these calculations.
For example, a carbon-12 nucleus contains 6 protons and 6 neutrons. It therefore has 12 nucleons. A carbon-14 nucleus also contains 6 protons, but it has 8 neutrons, so its number of nucleons is 14.
In the notation ¹⁴₆C, the upper number represents the mass number A, and the lower number represents the atomic number Z. To determine the composition of this nucleus:
- Read the number of protons: Z = 6.
- Subtract Z from A to find the number of neutrons: 14 − 6 = 8.
- Take A = 14 as the total number of nucleons.
The mass number is not the same as the mass of an atom. It is a count of particles, so it is an integer and has no unit. The atomic mass value given in the periodic table usually takes into account the isotopic composition of the element; it should not be automatically rounded to determine the composition of a specific nucleus.
What do nucleons have to do with elements and isotopes?
The number of protons determines which element we are dealing with. Every nucleus containing 6 protons is a carbon nucleus, and every nucleus containing 8 protons is an oxygen nucleus. Changing the number of protons means changing the element.
Isotopes are forms of the same element that have the same number of protons but differ in their number of neutrons. Carbon-12 and carbon-14 are therefore isotopes of carbon. A difference in the number of neutrons can affect the stability of the nucleus: carbon-12 is stable, whereas carbon-14 undergoes radioactive decay.
Not every nucleus contains both types of nucleon. The nucleus of the most common isotope of hydrogen, hydrogen-1, consists of just one proton. It has one nucleon and no neutrons.
What holds nucleons together?
Positively charged protons repel one another electrically. However, nuclei can remain bound because of the nuclear interaction between nucleons, which is a manifestation of the strong interaction. It acts over very short distances, comparable to the sizes of atomic nuclei.
Neutrons participate in this interaction without increasing the electrical repulsion between protons. However, this does not mean that adding neutrons always stabilises a nucleus. Both an excess and a shortage of neutrons relative to the number of protons can lead to nuclear instability.
The binding of nucleons is associated with binding energy—the energy needed to separate a nucleus into individual protons and neutrons. The mass of a bound nucleus is smaller than the sum of the masses of its free constituents. This difference corresponds to the binding energy according to the relationship E = mc².
Is a nucleon an elementary particle?
A nucleon is not an elementary particle—that is, a particle with no known internal structure. Protons and neutrons have an internal structure described in terms of quarks and gluons. Gluons carry the strong interaction between quarks.
In the basic description, a proton has three valence quarks: two up quarks and one down quark, while a neutron has one up quark and two down quarks. This simplified description is sufficient to explain the difference in their charges, although the full structure of a nucleon is more complex.
When solving a problem about the number of nucleons, start by finding the mass number A. If only the element symbol is given, without specifying the isotope, the exact number of nucleons usually cannot be determined.
See also: What Is BGP?
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