A nucleon is either a proton or a neutron, the two types of particles that make up the nucleus of every atom. The term has been in use since 1923 as a convenient way to refer to both particles collectively. Every element on the periodic table is defined by how many nucleons sit in its nucleus, making these particles fundamental to the structure of all matter.
Protons and Neutrons: The Two Nucleons
Protons carry a positive electrical charge of +1, while neutrons are electrically neutral. Despite that difference, they’re remarkably similar in size and mass. Both weigh approximately 1 atomic mass unit (amu), which translates to about 1.67 × 10⁻²⁷ kilograms. A neutron is slightly heavier than a proton, but the difference is so small it’s negligible for most purposes. Each nucleon measures roughly 0.83 femtometers in radius, a scale so tiny that a million nucleons lined up side by side wouldn’t span a single atom.
The number of protons in a nucleus determines what element an atom is. Hydrogen has one proton, carbon has six, gold has 79. Neutrons, meanwhile, act as stabilizers. Change the number of neutrons and you get a different isotope of the same element, some stable and some radioactive. The total count of protons plus neutrons gives you the mass number, which is why “nucleon number” and “mass number” mean the same thing.
What Nucleons Are Made Of
Nucleons aren’t truly fundamental particles. Each one is built from three smaller particles called quarks, held together by the strong force. A proton contains two “up” quarks and one “down” quark. A neutron flips that recipe: two down quarks and one up quark. The different quark combinations explain why protons are positively charged and neutrons are neutral. Up quarks carry a charge of +2/3, and down quarks carry -1/3, so the math works out to +1 for the proton and exactly zero for the neutron.
Because nucleons are made of three quarks, physicists classify them as baryons, a category of composite particles. All baryons are assigned a baryon number of 1, and nucleons have a quantum property called spin with a value of 1/2. These labels matter in particle physics because they dictate how nucleons interact and which reactions are possible.
What Holds Nucleons Together
Packing protons into a tiny nucleus creates an obvious problem: positive charges repel each other through the electromagnetic force. The reason they stay put is the strong nuclear force, which is far more powerful than electromagnetic repulsion but only works across incredibly short distances.
At the scale of a nucleus, the strong force operates through the exchange of particles called mesons between neighboring nucleons. You can picture it like two people tossing a ball back and forth. As long as the exchange keeps happening, the nucleons stick together. But the nucleons must be extraordinarily close for this to work, roughly within one proton diameter of each other. Beyond that distance, the strong force drops off so sharply that it effectively vanishes, and electromagnetic repulsion takes over.
This is why fusing nuclei together (as happens inside stars) requires extreme temperatures and pressures. The nuclei need to be moving fast enough to overcome their electrical repulsion and get close enough for meson exchange to kick in. Once they do, the strong force locks them together.
Binding Energy and Missing Mass
Something curious happens when nucleons combine into a nucleus: the resulting nucleus weighs less than the sum of its parts. This “missing” mass is called the mass defect, and it corresponds to the energy released when the nucleons bind together.
A copper-63 nucleus illustrates this well. If you add up the individual masses of its 29 protons and 34 neutrons, you get 63.506 amu. But the actual mass of the copper-63 nucleus is only 62.914 amu. The difference, 0.592 amu, has been converted into binding energy according to Einstein’s famous equation relating energy and mass. The greater the mass defect per nucleon, the more tightly bound and stable the nucleus is. Iron and nickel have the highest binding energy per nucleon of any elements, which is why they sit at the bottom of the nuclear energy curve and are produced in abundance inside massive stars.
Nucleon Stability
Inside a nucleus, both protons and neutrons can be perfectly stable, held in place by the strong force and the balance of energy within the nuclear structure. Outside a nucleus, though, the two particles behave very differently.
A free proton is stable. Left on its own, it will sit unchanged indefinitely (or at least for an extraordinarily long time; no proton decay has ever been observed). A free neutron is another story. Once a neutron is ejected from a nucleus, it survives for only about 880 seconds on average, roughly 14 and a half minutes. It then decays into a proton, an electron, and a nearly massless particle called an antineutrino. This process, called beta decay, happens because one of the neutron’s down quarks transforms into an up quark through the weak nuclear force, effectively turning the neutron into a proton.
This instability is why free neutrons don’t accumulate in nature. They either get absorbed into a nucleus quickly or decay into protons. Inside a stable nucleus, the energy balance prevents this decay from occurring, which is why neutrons can persist for billions of years as long as they remain bound.
Why the Concept Matters
The nucleon is a useful concept because protons and neutrons behave so similarly inside the nucleus. They have nearly identical masses, respond to the strong force in the same way, and can even transform into each other under the right conditions. Physicists treat them as two states of the same particle, distinguished mainly by electric charge. This symmetry, called isospin symmetry, simplifies calculations in nuclear physics enormously.
Nucleon count also determines practical things you encounter in everyday science. The mass number on a periodic table is the total number of nucleons. Radioactive dating works by tracking changes in nucleon composition over time. Medical imaging techniques like PET scans rely on nucleon interactions. From the periodic table on a classroom wall to the fusion reactions powering the sun, nucleons are the building blocks that make it all work.