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What determines whether a nuclear reaction releases energy or absorbs it?

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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.

Whether a nuclear reaction releases or absorbs energy is determined by the change in the total mass of the particles involved, a relationship governed by Einstein’s mass–energy equation, E = mc².12

A reaction releases energy (is exoergic) if the total mass of the resultant particles is less than the mass of the initial reactants. Conversely, if the total mass of the products is greater than that of the reactants, the reaction absorbs energy (is endoergic).3

This energy balance is often expressed through the Q-value:
* If Q > 0, the reaction is exoergic and releases energy.3
* If Q < 0, the reaction is endoergic and absorbs energy.3

In the context of nuclear fusion and nuclear fission, these processes release energy because the binding energy per nucleon increases. Because the binding energy per nucleon reaches a maximum near iron, the fusion of light elements (lighter than iron) or the fission of heavy elements generally results in a net release of energy.14