A nucleon is a particle found in an atomic nucleus, either a proton or a neutron. In AP Physics 2, nucleons are held together by the strong force, and every valid nuclear reaction (fission, fusion, or decay) must conserve total nucleon number.
A nucleon is the umbrella name for the two particles that live in the nucleus: protons and neutrons. When a question doesn't care which one it's talking about, it says "nucleon." The mass number A of an isotope is literally its nucleon count, so uranium-235 has 235 nucleons (92 protons plus 143 neutrons).
Two CED ideas hang on this word. First, nucleons interact through the strong force, which only acts at nuclear scales but dominates there, overpowering the electric repulsion between protons. Second, nucleon number is conserved in every nuclear reaction. Protons can turn into neutrons (and vice versa, as in beta decay), but the total count of nucleons before and after never changes. That conservation law, alongside conservation of charge, energy, and momentum, is what decides whether a proposed nuclear reaction is even possible.
Nucleons sit at the center of Topic 15.7 (Fission, Fusion, and Nuclear Decay) in Unit 15: Modern Physics. Learning objective 15.7.A asks you to describe the physical properties that constrain interacting nuclei and nucleons, and the essential knowledge spells those constraints out: the strong force governs nucleon interactions, nucleon number is conserved, and mass and energy can be exchanged via E=mc². Almost every nuclear reaction problem on the exam is really a bookkeeping problem about nucleons. Balance the mass numbers, balance the charges, then use the mass difference to find the energy released. If you can't count nucleons cleanly, you can't do Topic 15.7.
Keep studying AP® Physics 2 Unit 15
E=mc² and mass-energy equivalence (Unit 15)
A nucleus weighs less than its separated nucleons. That missing mass is the binding energy, converted via E=mc². Fission and fusion release energy because the product nucleons end up more tightly bound, even though the nucleon count itself never changes.
Conservation of mass vs. conservation of nucleon number (Unit 15)
In nuclear reactions, mass alone is NOT conserved (some becomes kinetic energy or photons), but nucleon number always is. This is the swap you have to make mentally when you move from chemistry-style thinking into Unit 15.
Radioactive decay and N = N₀e^(-λt) (Unit 15)
Decay is a nucleus rearranging or converting its nucleons. In beta-minus decay a neutron becomes a proton, so the mass number A stays put while the atomic number Z goes up by one. The exponential decay law then tells you how many undecayed nuclei remain over time.
Annihilation (Unit 15)
Annihilation is the contrast case. When a particle meets its antiparticle, both vanish into photons, so this is the one process where talking about "conserving particles" breaks down and you lean entirely on energy and momentum conservation instead.
You won't get asked "define nucleon." You'll get asked to use it. Multiple-choice stems hand you a nuclear reaction and ask what constrains it. For example: why must beta-minus decay emit both an electron and an antineutrino (conservation laws), what limits two protons fusing in the Sun's core (electric repulsion that the strong force must overcome at close range), or why fission fragments of uranium-235 are more stable than the parent nucleus (the nucleons are more tightly bound, so mass converted to energy). The core skills are balancing nucleon number and charge on both sides of a reaction, and connecting any mass difference to released energy through E=mc². No released FRQ has demanded the word "nucleon" itself, but reaction-balancing and energy-release reasoning are standard Unit 15 territory.
A nucleon is one particle (a single proton or neutron); the nucleus is the whole cluster of nucleons bound together at the atom's center. Helium-4 has one nucleus made of four nucleons. Mixing these up garbles conservation statements, since it's nucleon number, the count of particles, that's conserved in reactions.
A nucleon is any particle in the nucleus, so the term covers both protons and neutrons.
The mass number A of an isotope equals its total nucleon count, which is why it's also called the nucleon number.
Total nucleon number is conserved in every nuclear reaction, even when a neutron converts into a proton during beta decay.
The strong force dominates nucleon interactions at nuclear scales, overcoming the electric repulsion between protons.
A nucleus has less mass than its separated nucleons, and that mass difference equals the binding energy through E=mc².
To check a nuclear reaction on the exam, balance nucleon number and charge first, then use the mass change to find the energy released.
A nucleon is a particle inside the nucleus, meaning either a proton or a neutron. The term shows up in Topic 15.7, where the CED says the strong force dominates nucleon interactions and nucleon number is conserved in nuclear reactions.
No. Electrons orbit outside the nucleus, so they don't count as nucleons. Even the electron emitted in beta-minus decay isn't a nucleon; it's created in the decay, and the nucleon count (mass number) stays the same.
No. In beta-minus decay a neutron converts into a proton, so one nucleon type changes into another but the total count is unchanged. The mass number A stays the same while the atomic number Z increases by one.
A nucleon is a single proton or neutron; a nucleus is the entire bound collection of them. Uranium-235 is one nucleus containing 235 nucleons (92 protons and 143 neutrons).
From mass, not from losing nucleons. The product nuclei have the same total nucleon count but slightly less mass, because their nucleons are more tightly bound. That mass difference becomes kinetic energy or photons via E=mc².
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