What is fission?

• A. The joining of atomic nuclei to form heavier elements
• B. The decay of radioactive material
• C. The emission of photons from excited electrons
• D. The process of splitting atoms in a chain reaction to release energy

Answer: (D)

Fission is the splitting of a heavy atomic nucleus into smaller nuclei, typically releasing energy and neutrons. When those neutrons go on to cause further fissions in nearby nuclei, a chain reaction happens, releasing energy rapidly. That combination—splitting atoms in a chain reaction to release energy—is the essence of fission.

For context, this process is seen with heavy elements like uranium-235 or plutonium-239. The energy comes from the mass difference between the original nucleus and the fission fragments, and the emitted neutrons can sustain the reaction if there’s enough material (critical mass).

The other descriptions refer to different phenomena: joining nuclei to form heavier elements is fusion; radioactive decay is the spontaneous breakdown of a nucleus over time; and emitting photons from excited electrons involves electronic transitions, not nuclear fission.