The energetic combination of atoms is known as nuclear fusion. In this process, two relatively light elements merge to form a new, heavier element amid the release of energy.
The new element formed by fusion has a mass somewhat lighter than the sum of the two elements that produced it. The lost mass is converted to energy in a relationship described by Einstein's famous E=mc2 equation. Atomic fusion generates energy for all elements lighter than iron. Since the fusion of iron nuclei requires more energy than it releases, iron reactions are not sustainable. According to Georgia State University, elements heavier than iron can release energy by splitting, a process known as fission.