Nucleons are the particles that make up an atomic nucleus: protons and neutrons. Every atom you’ve ever encountered, from the hydrogen in water to the iron in your blood, has a nucleus built from these two types of particles packed tightly together. Understanding nucleons means understanding what holds matter together at its most fundamental level.
Protons and Neutrons: The Two Nucleons
Protons carry a positive electric charge, while neutrons are electrically neutral. That’s the most obvious difference between them, but they’re remarkably similar in almost every other way. A proton has a mass of about 1.673 × 10⁻²⁷ kilograms, and a neutron is just slightly heavier at roughly 1.675 × 10⁻²⁷ kilograms. The neutron outweighs the proton by about 0.14%, a tiny difference that has enormous consequences for how atoms behave.
Both particles are incredibly small. The proton’s radius has been measured at 0.833 femtometers (trillionths of a millimeter), and the neutron is comparable in size. To put that in perspective, if an atom were the size of a football stadium, the nucleus containing all its nucleons would be roughly the size of a marble at the center.
Each nucleon is itself made of smaller particles called quarks, held together by carriers of force called gluons. A proton contains two “up” quarks and one “down” quark. A neutron contains one up quark and two down quarks. This internal structure is what gives them their different charges and slightly different masses.
What Holds Nucleons Together
Here’s the puzzle: protons are all positively charged, and positive charges repel each other. So why doesn’t every nucleus fly apart? The answer is a force far more powerful than the electrical repulsion between protons. The strong nuclear force, as physicists call it, is roughly ten times stronger than electromagnetism at nuclear distances.
The strong force primarily works between the quarks inside each nucleon, binding them together through the exchange of gluons. But there’s a residual version of this force that leaks out and acts between neighboring nucleons, pulling them toward each other. Lawrence Berkeley National Laboratory describes the nucleus as a kind of “strong force molecule,” comparing it to how atoms form chemical bonds. In a molecule, the electrical force binds electrons to individual atoms, but a leftover interaction between electron clouds can glue atoms together. Similarly, the strong force binds quarks inside nucleons, and its residual effect binds nucleons inside nuclei.
This residual force has an extremely short range, only about 1.5 × 10⁻¹⁵ meters, roughly the diameter of a single nucleon. That’s why it only works when nucleons are packed essentially side by side. It also explains why very large nuclei (like uranium) become unstable: nucleons on opposite sides of a big nucleus are too far apart to feel the strong force pulling them together, but they still feel the electrical repulsion pushing them apart.
Binding Energy and the Mass Defect
One of the strangest facts about nucleons is that a nucleus always weighs less than its individual protons and neutrons would if you weighed them separately. The missing mass, called the mass defect, has been converted into the energy that holds the nucleus together. Einstein’s famous equation, E = mc², describes this conversion precisely. The energy equivalent of that missing mass is called the nuclear binding energy.
Binding energy per nucleon varies depending on the element. Light elements like hydrogen and helium have relatively low binding energy per nucleon. As you move up the periodic table, binding energy per nucleon increases, peaking around iron and nickel. Nickel-62 is actually the most tightly bound nucleus, at just over 8.8 MeV per nucleon, with iron-56 close behind. After that peak, binding energy per nucleon gradually decreases for heavier elements.
This curve explains both nuclear fusion and nuclear fission. Fusing light nuclei (like hydrogen into helium) moves you up the curve toward the peak, releasing energy. Splitting very heavy nuclei (like uranium) also moves you toward the peak from the other direction, again releasing energy. Both processes work because the resulting nucleons end up more tightly bound than they started, and that difference in binding energy is what comes out as usable power.
Nucleon Stability
Protons are extraordinarily stable. No experiment has ever observed a proton decaying, and current estimates place its lifetime at well beyond 10³⁴ years, far longer than the age of the universe.
Neutrons are a different story. A free neutron, one that’s been knocked out of a nucleus, will decay with a half-life of about 10.3 minutes. It transforms into a proton, an electron, and a tiny particle called an antineutrino. This decay involves a quark-level transformation: one of the neutron’s down quarks converts into an up quark through the weak nuclear force, turning the neutron into a proton.
Inside a nucleus, however, neutrons can be perfectly stable. The binding energy of the nucleus effectively prevents the decay from occurring because there isn’t enough available energy for the transformation to happen. This is why atoms with balanced numbers of protons and neutrons can persist indefinitely. Going the other direction is also possible: a proton can transform into a neutron, but only if at least 1.29 MeV of energy is supplied to make up the mass difference. This process occurs in certain unstable nuclei and inside stars.
Why the Number of Nucleons Matters
The total number of nucleons in a nucleus is called the mass number, written as “A” in physics notation. Carbon-12, for example, has 12 nucleons (6 protons and 6 neutrons). This number essentially determines an atom’s mass, since electrons contribute less than 0.05% of the total weight.
Atoms of the same element always have the same number of protons but can have different numbers of neutrons. These variants are called isotopes. Carbon-12 and carbon-14 are both carbon (6 protons each), but carbon-14 has two extra neutrons. Those extra neutrons change the nuclear binding energy balance, which is why carbon-14 is radioactive while carbon-12 is stable.
The ratio of protons to neutrons in a nucleus determines its stability. Light, stable nuclei tend to have roughly equal numbers. Heavier stable nuclei need progressively more neutrons than protons, because the extra neutrons contribute strong-force attraction without adding electrical repulsion. Lead-208, one of the heaviest stable nuclei, has 82 protons but 126 neutrons. Beyond a certain size, no combination of protons and neutrons produces a stable nucleus, which is why all elements heavier than lead are radioactive.