Final answer:
The fusion of light nuclei into heavier nuclei releases energy because it converts mass into energy, following Einstein's equation E = mc², where the resulting nucleus has less mass than the sum of the original nuclei, and this missing mass is converted into energy.
Step-by-step explanation:
The fusion of light nuclei into heavier nuclei releases energy because this process converts mass into energy. This is explained by the principle that energy is released when light atomic nuclei join together to form a heavier one. According to Einstein's famous equation E = mc², the energy (E) produced is proportional to the mass (m) that is converted to energy, times the speed of light (c) squared. During nuclear fusion, the combined mass of the resulting nucleus is less than the sum of the masses of the original nuclei, and this "missing mass" is what is converted into energy.
The binding energy per nucleon (BE/A) is lower for the lighter nuclei compared to the heavier nuclei they form. This increased binding energy in the product nucleus results in a more stable nucleus and a release of energy as the nucleons are in a lower energy state. The sun's energy production through fusion of hydrogen into helium is a natural example of this process.