The binding energy per nucleon curve is pivotal in understanding why energy is released in nuclear fission and fusion. This curve shows that nuclei with intermediate mass numbers (like iron and nickel) have the highest binding energy per nucleon, indicating they are the most stable. In fission, a heavy nucleus (with a lower binding energy per nucleon) splits into smaller nuclei that are closer to the peak of this curve, resulting in nuclei with higher binding energy per nucleon. This increase in binding energy per nucleon means that energy is released during the process. Conversely, in fusion, light nuclei (with relatively low binding energy per nucleon) fuse to form a heavier nucleus with a higher binding energy per nucleon, closer to the curve's peak. Again, this transition to a state of higher binding energy per nucleon releases energy. Thus, both processes release energy as they move towards a state of greater nuclear stability, as indicated by the binding energy per nucleon curve.

While the energy-mass equivalence principle (E = mc²) is universally applicable, its impact in chemical reactions is negligible due to the extremely small scale of mass changes. Chemical reactions involve the rearrangement of electrons in atoms and molecules, without significant changes in nuclear mass. The energy changes in chemical reactions are the result of differences in electronic energy levels, not nuclear transformations. Thus, any mass change is so minuscule that when applied in the E = mc² equation, the resulting energy change is insignificant compared to the total energy changes observed in chemical reactions. Therefore, in practical terms, chemical reactions are typically analyzed using principles of chemical energetics, such as bond energies and enthalpy changes, rather than nuclear physics.

Energy release in nuclear reactions like fusion and fission is fundamentally due to the principles of nuclear binding energy and the energy-mass equivalence. In these reactions, the total binding energy of the products is greater than that of the reactants. In fission, when a heavy nucleus splits into lighter nuclei, the lighter nuclei have a greater binding energy per nucleon, meaning they are more stable and thus, energy is released to reach this more stable state. Similarly, in fusion, when light nuclei combine to form a heavier nucleus, the resulting nucleus has a higher binding energy per nucleon, and thus, is more stable. The difference in binding energy manifests as the release of energy, according to E = mc², where the mass defect (the difference in mass between reactants and products) is converted into energy. This release of energy is a natural process as systems tend to move towards a state of lower energy and greater stability.

The extraordinary energy yield in nuclear reactions, as compared to chemical reactions, is primarily due to the energy-mass equivalence principle encapsulated in the equation E = mc². In chemical reactions, the energy changes involve the electrons of atoms, typically leading to relatively small energy releases because the involved mass changes are minute. On the other hand, nuclear reactions involve changes in the nucleus of atoms, where even a minuscule change in mass can result in immense energy release due to the multiplication by the square of the speed of light (c²), which is a very large number (approximately 9 x 10¹⁶ m²/s²). For example, in fission or fusion, the mass defects (the difference between the mass of the reactants and products) though small in absolute terms, when multiplied by c², yield vast quantities of energy. This explains why nuclear power plants, which exploit fission reactions, can produce so much more energy than traditional chemical-based energy sources like fossil fuels, from a comparably small amount of fuel.

Harnessing energy from nuclear fusion presents greater challenges compared to nuclear fission due to several factors. Firstly, achieving the conditions necessary for fusion – extremely high temperatures and pressures – is technologically demanding. Fusion requires temperatures of millions of degrees to overcome the electrostatic repulsion between positively charged nuclei. Such conditions are hard to create and maintain on Earth. Secondly, containing the plasma (a state of matter at high temperatures where electrons are stripped from atoms) in a stable state requires advanced magnetic confinement techniques, such as those used in tokamaks or inertial confinement methods. These technologies are still under development and require significant investment and research. In contrast, nuclear fission, used in current nuclear power plants, occurs at much lower temperatures and is easier to sustain and control. The technology for fission is well-established, making it currently more feasible for energy production compared to fusion, which remains largely experimental and in the research phase.