Binding energy
Also called Nuclear binding energy
Binding energy is the energy needed to pull a bound system apart. AP Physics 2 defines it for an atom, as the energy required to remove an electron, and then uses it for a nucleus, as the energy required to separate the nucleons.
The CED prints exactly one definition of binding energy, and it is atomic rather than nuclear. Essential knowledge 15.3.A.5: binding energy is the energy required to remove an electron from an atom, causing the atom to become ionized, and an atom in the lowest energy level will require the greatest amount of energy to remove the electron.
The term then reappears at 15.7.A.8, where fission may occur spontaneously or may require an energy input depending on the binding energy of the nucleus. That nuclear use is never defined. The idea is the same one level down: the energy required to separate a nucleus into its individual nucleons.
What makes it measurable is . A bound nucleus has less mass than the sum of its separated nucleons, and that shortfall is the mass defect. Multiply the mass defect by and you have the binding energy. The CED does not use the phrase mass defect anywhere, so define it before leaning on it in an answer.
Binding energy per nucleon is the quantity that decides outcomes. Total binding energy rises simply because a large nucleus has more nucleons; what settles whether a reaction releases energy is the change per nucleon. Products bound more tightly per nucleon than the reactants means the total mass falls, which is why fission of heavy nuclei and fusion of light ones can both release energy.
Neither a binding-energy curve nor a table of nuclear masses appears anywhere in the AP Physics 2 reference material.