nuclear fission

Meaning: In fission, a heavy nucleus such as ²³⁵U absorbs a neutron, becomes unstable, and splits into two medium-mass fragments plus a few free neutrons. The fragments have a higher binding energy per nucleon than the parent, so they sit closer to the peak of the binding energy curve (near iron). Their total mass is slightly less than the starting mass, and this mass defect appears as kinetic energy and radiation. The released neutrons can trigger more fissions, which is a chain reaction. Reactors control it with neutron-absorbing control rods; weapons do not.

Equation: E = Δm c², where E is energy released (J), Δm is the mass lost (kg), and c = 3.00 × 10⁸ m/s. Example: ²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n + energy.

On the MCAT: You may balance a nuclear equation by conserving mass number and atomic number, find a missing fragment or neutron count, read the binding energy per nucleon curve, or use Δm in E = Δm c². Reactor passages ask what happens to the rate if control rods are inserted.

Common confusion: Fission splits heavy nuclei, while fusion joins light nuclei. Both release energy because the products are more tightly bound per nucleon. Mass number is conserved in the equation even though a small amount of mass is lost.