fission

Meaning: Fission is the splitting of a heavy nucleus, such as uranium-235, into two mid-sized nuclei, usually after it absorbs a neutron. The products have higher binding energy per nucleon than the parent, so their total mass is slightly less, and the difference is released as energy. Released neutrons can cause more fissions, which is a chain reaction. (Binary fission in biology is bacterial cell division, a different idea.)

Equation: E = Δm c², where E is the energy released (J), Δm is reactant mass minus product mass (kg), and c = 3.00 × 10⁸ m/s. Example: ²³⁵U + n → ¹⁴¹Ba + ⁹²Kr + 3n.

On the MCAT: Expect balancing a nuclear equation (mass numbers and atomic numbers each sum equally on both sides), reading a binding energy per nucleon curve to explain why heavy nuclei release energy by splitting, or estimating energy from a mass defect.

Common confusion: Fission and fusion are easy to swap. Fission splits heavy nuclei and fusion joins light ones; both move products toward the peak of the curve near iron-56. Also, mass number is conserved in fission; it is the rest mass that decreases slightly.