Also referred to as nuclear fission, splitting an atom results in its overall mass being reduced, causing the release of a relatively massive amount of energy. Virtually every atom can generate nuclear energy in this way, but those with the greatest atomic mass will provide the most energy from fission.
Nuclear reactors rely on fission to generate heat, which then causes cooling pools to boil, driving steam turbines. In other cases, it is the sudden and violent fission process that results in nuclear detonations that can devastate huge geographic areas. The process itself is remarkably efficient, but requires a large investment of initial energy to generate. In opposition to fission, the fusion process by which two atoms are joined generates several times as much energy, but does not present the risk of nuclear contamination or pollution. As well, spent isotope fuel rods remain hazardous for many thousands of years while the materials consumed during the fusion process remain safe to handle and transport. The most common fusion process, that of converting hydrogen and oxygen into water, is often the easiest and most frequently referred to. Not only are these two elements the most plentiful, they also join more readily than other elements.