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Why can fusion release energy for elements lighter than iron?

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laser-activated fusion — nuclear fusion Interior of the U.S. Department of Energy's National Ignition Facility (NIF), located at Lawrence Livermore National Laboratory, Livermore, California. The NIF target chamber uses a high-energy laser to heat fusion fuel to temperatures sufficient for thermonuclear ignition. The facility is used for basic science, fusion energy research, and nuclear weapons testing.

Fusion releases energy for elements lighter than iron because of the behavior of nuclear binding energy. Binding energy is the energy required to separate a nucleus into its constituent protons and neutrons, or conversely, the energy liberated when these nucleons combine.12

The efficiency with which nucleons are bound together is measured by the binding energy per nucleon. This value is not constant across the periodic table; it increases as nuclei grow larger until it reaches a maximum in the vicinity of iron.13

When lighter elements undergo nuclear fusion, the resulting nucleus is more tightly bound than the original, lighter nuclei. Because the new nucleus has a higher binding energy per nucleon, the process results in a net release of energy, which is accounted for by a small decrease in total mass—a phenomenon explained by Albert Einstein’s mass-energy equivalence equation, $E = m c^{2}$ .123