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How is nuclear fission different from nuclear fusion?

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nuclear fission Italian-born physicist Enrico Fermi explaining a problem in physics, c. 1950. He was awarded the 1938 Nobel Prize for Physics.

Nuclear fission and nuclear fusion are both processes that release significant amounts of nuclear energy, but they function through opposite mechanisms:

Nuclear Fission: This process involves the splitting of a heavy atomic nucleus (such as uranium or plutonium) into two lighter nuclei of roughly equal mass. Because neutrons have no electric charge, they can easily enter a nucleus to induce this split, even at room temperature. This reaction can trigger a chain reaction, which is utilized in nuclear reactors for power or in atomic bombs for destruction.123
Nuclear Fusion: This process involves joining light atomic nuclei (such as hydrogen isotopes) to form heavier elements. Unlike fission, fusion must overcome the Coulomb barrier—the electrostatic repulsion between positively charged nuclei. Consequently, fusion requires extremely high temperatures to reach a plasma state, making it much more difficult to harness for practical electricity production than fission.143