Eitan reads two headlines. One says a reactor split heavy nuclei; the other says a machine merged light ones. Both end the same way: energy out. Telling the two reactions apart takes one look at the nuclei.
You met both definitions. Fission: a heavy nucleus splits into two smaller nuclei, releasing neutrons and energy. Fusion: two light nuclei merge into one heavier nucleus, releasing energy. This lesson makes the verdict automatic.
To classify a nuclear reaction, ask one question: what happened to the nuclei?
One heavy nucleus splitting into smaller pieces is fission.
Light nuclei merging into one heavier nucleus is fusion.
Energy release does not decide the verdict: both fission and fusion release energy.
Released neutrons do not decide it either: fissions release two or three, and some fusions release one or two as well.
Inside a star, two helium-3 nuclei combine into one helium-4 nucleus plus two protons. Is this reaction fission or fusion?
Ask what happened to the nuclei.
Two light nuclei merged into one heavier nucleus.
Light nuclei merging into one heavier nucleus is fusion.
This reaction is fusion.
A uranium-235 nucleus absorbs a neutron and splits into a barium nucleus and a krypton nucleus, releasing three neutrons. Is this reaction fission or fusion?
Ask what happened to the nuclei.
One heavy nucleus split into two smaller nuclei.
One heavy nucleus splitting into smaller pieces is fission.
This reaction is fission.
A plutonium-239 nucleus absorbs a neutron and splits into two smaller nuclei, releasing three neutrons. Is this reaction fission or fusion?
Two deuterium nuclei collide and merge into one helium-3 nucleus, releasing a neutron. Is this reaction fission or fusion?
A deuterium nucleus merges with a helium-3 nucleus, making one helium-4 nucleus and releasing a proton. Is this reaction fission or fusion?
A uranium-235 nucleus absorbs a neutron and splits into a xenon-140 nucleus and a strontium-94 nucleus, releasing two neutrons. Is this reaction fission or fusion?
A very heavy nucleus splits into two smaller nuclei and two neutrons without absorbing anything first. Is this reaction fission or fusion?
Two tritium nuclei merge into one helium-4 nucleus, releasing two neutrons. Is this reaction fission or fusion?
Which of the following nuclear reactions is fission?
Inside a star, a helium-3 nucleus merges with a helium-4 nucleus, making one beryllium-7 nucleus. Which kind of nuclear reaction is this?
A reaction is classified as fission: a uranium-235 nucleus ends as two smaller nuclei of different elements, with neutrons and energy released. Which feature identifies the reaction as fission?